Here's some background on why I'm even answering this question: Jessica says she doesn't give blow jobs here: Jessica Su's answer to What is your biggest strength and weakness regarding relationships? We had a short conversation there regarding blowjobs and she asked me to answer the question more thoroughly here.
I used to not give blow jobs. Adamantly. Even when I was married. I think I understand the reasons behind some of the hesitation and I am writing this in the hopes that somebody somewhere has a more fulfilling sex life rife with dopamine and lots of mutual understanding because of it.
Giving blowjobs is intimidating for lots of women. We've been brought up to believe it's yucky down there and good girls don't do that. Even those of us who are sexually active, sometimes with multiple partners, still don't want to or don't know how to give blowjobs. I was sexually active for 10 years before I gave my first and it was another year before I was interested enough to try it again (different partner).
My answer isn't going to focus on the details of any man's specific preferences – that totally varies other than a couple consistent techniques, but rather a way to become accustomed to and to enjoy the experience as a woman.
First and foremost I have to recommend that you find a partner whose pheromones turn you on. This is totally underestimated by a lot of people. I think it's because they haven't experienced it before.
If you've experienced this, you know what I mean. If not, the best way I can explain it to you is that when your mate is freshly showered, their genitals smell comforting, stimulating, and mouth watering. You want to inhale deeply.
I think it's always good to start out slowly and explore, especially if you're totally uncomfortable with it like I was. You don't have to start out deepthroating like a pornstar (though this doesn't hurt later on).
Flavored condoms can work for the first couple of times if you are really uncomfortable – I did it, but I didn't like it. You could keep your mouth closed and just smell, feel skin on skin, kiss, nuzzle, etc. Get used to how it smells and how it looks. Hopefully at this point you've discovered that you like how it looks, smells, and feels.
You can continue to move up from there – lose the condom, begin to use your hands, mouth, and tongue in different ways. This doesn't mean it even has to be in your mouth yet. You can just slowly explore. If there's hair, you can touch, feel, and taste that. Same with the upper thighs, lower abdomen, hips, buttocks, scrotum, balls, and taint (or even anus once/if that time comes). If you haven't explored the nipples yet, this could be a good time for that too. If you have a patient and loving partner I see no reason why they would object and would probably enjoy teaching and guiding someone to do things the way they like – maybe they're learning too. Ask for feedback. Try specific things and get feedback. It may seem unromantic or embarassing, but the next time you put your new found knowledge to good use it will all be worth it.
Once you get to the enveloping stage, there's a couple more things to remember. First, use (and alternate) your hands and LOTS of spit. I find drinking water beforehand and keeping a cup nearby for extended sessions helps a lot. Then you've got to figure out what to do about your teeth. I was really worried about scraping, but for the most part lightly covering the edges of your teeth with your lips seems to work alright. Different anatomies might be different. You should also have a suction effect as the penis leaves your mouth.
Deepthroat. Honestly, if you would have told me five years ago that I would do something like that I would have just laughed at you. But I hacked it. Yeah, I still have a gag reflex but it can be mitigated. Some people recommend desentization with bananas or cucumbers but I've found something else that works.
I've been practicing yoga for about 10 years so I have pretty good control of muscles in my body that most other people don't simply because they don't practice. The throat houses some of those muscles. The specific breath I used is ujjayi. If you're interested, here's a brief pranayama instruction (you might learn better from YouTube videos, so you can try those too). You just can't see the neck muscles move so I think it's better to have a written description:
The Victorious Breath
Once you can practice samavrtti with ease for 10 to 15 minutes, you can move on to the practice of Ujjayi Pranayama (Victorious Breath). Ujjayi is simply doing samavrtti with the addition of a slight closure at the root of your throat. Narrowing the throat by half-closing the epiglottis (the piece of cartilage at the top of your voice box) gives your breath a voice. Let that voice become your teacher. Listen to the tone of that voice as you inhale and exhale, and make that tone as even and smooth as you can, without any catches or wavering and without any change in pitch. Listening to the voice of ujjayi pranayama will give you greater sensitivity and control over the nuances of your breath.
At first, you may wonder exactly how to manipulate this epiglottal valve at the root of your throat. Here are two methods which can help you learn this action. First, just sigh, and notice the slight constriction in your throat that occurs. That's the area you need to control when you're practicing ujjayi. A second way is to open your mouth and inhale softly, noticing where the breath touches your throat. For most people, that will be deep down at the base and back of the throat. Again, that's the spot you need to constrict slightly to practice ujjayi. After you've zeroed in on this area, close your mouth and inhale, letting the breath touch your throat there. Once you can inhale in this way, practice exhaling with the same constriction of the epiglottis.
Once you've mastered that constriction, you basically use those same muscles to open up and relax your throat. If you have any other personal relaxation techniques you can use, you may want to try those out too. I basically use deep breathing through my nose while manually expanding my throat muscles any time I choose to deep throat. It takes a lot of concentration and effort, but this is one of the "techniques" that reaps the greatest rewards. I'm not sure why because I can't feel it but I definitely know there's a different physical and audible response in my partner. You can even just try slowly working your way down over time until you find something that works for you and your partner, again – communication, communication, communication.
But what about when he cums? Swallow or spit? What's it like? Does it happen everytime? Personally, I knew if I was in, I was all in. Taste isn't bad. Smell is disgusting. Sorry guys, but salty bleach smell stuck in the back of my throat and wafting through my nose is not my favorite! I don't think spitting at this point makes a difference – I never have so I don't know the proper disposal techniques for that. Perhaps someone else can chime in.
I generally don't breathe in my nose until I swallow and drink a lot of water. A couple seconds of minor unpleasantness is worth it for me. Pineapples (I've heard strawberries recently, too) help to make the taste and smell more neutral. Plus I know when the tables are turned, it's not ALL roses and I'd like someone to stick around until I'm finished too. It also has a numbing effect (similar to Novocain) on anything it touches in the mouth or throat. This was surprising but it passes in just a few minutes.
You don't have to though. While women can have both oral sex and intercourse in the same session, it's much harder for men. I would say a majority of the time, you won't have to worry about cum in your mouth – never if you're really opposed.
69s can be fun too, but in a different way. There's not as much focus on one person's anatomy and preferences, but you can develop a certain level of non verbal communication and synergy that is pretty amazing. Takes time and practice but so, so, SO worth it.
I want women to feel empowered, confident, and to explore their own sexuality. It's fluid and as you learn more about what you are doing, things that seemed gross, scary, uncomfortable, or weird become some of the most powerful tools in your belt.
Saying I don't give blowjobs is like saying I don't do math. It's more a matter of being uncomfortable with the material because you don't know how to manipulate the variables than it is true unwillingness.
Knowledge is power, even (especially) in the bedroom.
Just to clarify once more since I have the feeling this may be a very interesting answer for some folks – I don't care how you like it or want it or if you don't like how I do it. Don't comment with any of that nonsense. Seriously. Go write your own answer! However, if you have further questions, need clarification, or want to piggyback on a part of my answer, comment on!