First, if she's pulling hard enough to hurt herself she should be on a harness. I prefer to teach all leash skills on a harness first because of this- puppies have delicate throats and can hurt themselves if they're constantly choking themselves in their excitement to see the world.
I would recommend a front-clip harness, personally, because IME back clip ones can often incite more pulling and give the dog better leverage, which with a strong dog (especially a bully breed; I'm assuming she's the puppy in you profile pic and that's what she is based off her looks) that's going to be touch on you and not super safe, either, once they're big enough to drag you off balance. Front-clip harness are often sold as "no pull" harnesses, but IME they rarely actually make a dog less likely to pull. I like them because 1) they give the handler much better leverage than a back-clip harness, 2) they're safer for the dog than a collar (although their have been studies that suggest they may cause gait disruption, so I wouldn't suggest them for anything more than hiking; get a back clip harness for hiking/jogging/etc), and 3) they tend to turn the dog back towards you the more they pull, which is helpful when you're in the early stages of training loose leash walking.
She is pulling because she doesn't understand what you want from her. To do that you need to train her what you don't want (ie, you don't want her to pull so that the leash is taut or to forge out away from you) and also what you do want (ie, the position she needs to be in, when she's allowed to sniff, how far from you she's allowed to be, etc).
First, you the same length leash for all walks. It's confusing for a dog to one day have 6 feet of leash and then the next have 4 or 5. She needs a constant amount of "bubble space" so she can solidify an understanding of how far from you she can go without hitting the end of the leash. If you don't want her moving at all from you heel, I'd suggest a leash on the shorter end of that. Do not use any extendable leashes- those train the dog that there will always be tension on the line. If you want to give her a little more freedom, use a long line.
In order to train her what not to do, I'd suggest the "Be a Tree" and "Penalty Yards" methods. For "Be a Tree", every time she pulls, you stop and don't move until she moves back towards you/looks at you/removes the tension from the line/comes back into heel position- decide what you criteria for this will be. Personally, I started with my dog needed to look at me and make some small movement to relax the tension on the line, now I wait for her to come back to heel position before we walk again. For "Penalty Yards", when the dog pulls, you move back a few steps in the opposite direction. In both cases, I have found the dog understands what you want quicker and is more excited to comply if you use a food reward. You can do it without treats but I would do it with them, because training is always better with treats! (and also because with a young distractable puppy its easier to have something to prove to them that paying attention to you and staying near you instead of smelling around and exploring at the end of the leash is worth it)
To train her what you do want, I would recommend you reward and praise heavily for being in the right position- wherever you decide that is. For me, if I'm on a walk, I want my dog in a decent heel close by me and don't like them to sniff around randomly unless I give them the OK, because I take her a lot of places and sometimes we're in a hurry to get somewhere. If I'm OK with her just meandering around she has a cue for that ("OK, Free"), and she also has sniffing on a cue ("Sniff"), so that if I'm in a hurry I'm not bothered by her head being on the pavement or her lunging after a fun scent. I know the places in our neighborhood dogs like to sniff and make sure to give her a chance to sniff there during our walk. For teaching position- keep up a steady stream of feedback in the beginning. If she's getting out of position, help her back into it with some verbal feedback/kissy sounds/whatever. The more you reward her for being in the right place on a walk with food, the more exciting being in the right place will become for her. My dog gets PUMPED when I put a leash on and literally jumps into a heel position, because for the last few months I've worked on feeding every step, then every two steps, then every three, and making it a really FUN place to be. It may also be helpful to train a nose-to-hand target to help with position; IMO its a good way to lure a dog back into the position you want them without actually luring them. If you look up how to train a competition level heel you can see this technique in action, although for a daily walk obviously it would be a little bit more relaxed. Also, throwing in random targets on a walk is also a great way to play some brain games.
If you are interested in training an automatic sit (ie, when you stop the dog sits), that is also fairly easy to teach, although it can take awhile for the dog to understand what you want. Personally I prefer my dogs to do this on walks because I live in a NYC neighborhood with relatively narrow streets and a lot of foot traffic and this helps to get her out of the way on walks- that's another reason I like an automatic close heel on a walk, as well. How I train it is every time you stop, you say "sit" just as you come to a stop. I tend to come to a very exaggerated stop in the beginning (ie, step forwards with the leg on the side they're on and slowly bring the other one next to it) so that its very clear that I'm stopping. I then treat as soon as they sit, and then release them (I use either "forwards", "back", or "OK!" depending on the situation and if the direction we're going) and continue walking. When I first teach them, I stop and start a lot on walks and every time I'm about to stop will give the sit command just as I come to an exaggerated stop. I now have a dog who is almost tripping over herself to sit before I stop, it's hilarious to watch and I've actually been tricked into thinking she had a limp a few times because she was walking strangely until I realized it was because I was walking slowly and every step she thought I was about to stop and so would start to sit and then realize we were still going, lol.
I personally have tried to be as force-free as possible in training leash skills. Some people like to use leash-corrections to teach leash skills, but personally I don't really like to do so. This is a force-free forum, so most of the people will probably say the same, although not everyone here is really force-free. I will say that it is important to understand stress signals and dog body language, as well as be very clear on what you're trying to achieve, if you do decide to use corrections in training leash skills. I would encourage you to at least try without corrections in the beginning and see how it goes for a month or two.
I'd use a harness (using the front clip) while you are training her not to pull. Once she does not pull later on down the road, you may be able to switch back to the normal collar...or even stay using a harness.