Namaste Ell!
Of course, as others have already posted, it's best to take a class with a qualified teacher, but I have a couple suggestions for you if you feel it's not possible to go that route.
First, could you possibly schedule a one or two time private session with a teacher? You could go over your regular asana practice with them, and just ask them to observe and critique you. Quite often, we feel like we are doing a posture correctly, but in reality we are not. I'll give you an example from my own life.
Once, my dad and I were debating the benefits of yoga. He doesn't know much about it and thinks it's mostly "fancy stretching". I showed him Downward Dog, explaining that the goal is to have the feet flat on the floor. He said "Oh that's so simple!" and immediately took this exact pose:

When I tried to explain how that differed from this:

it was very difficult for him to understand what he was doing differently. To him, what he was doing felt like it was the same as what I'd shown him. It wasn't until I got my strap out and pulled his hips into the correct spot and pushed his head down, etc, that he finally got it. Once you have some idea of what the postures should feel like, then practicing on your own will probably be more productive.
This is why I think you should pay for one or two private sessions. You can have the instructor's full attention, and get the most out of the time. If it were me, I'd explain my purpose to the instructor upfront. "I'm here to get a foundation so I can practice alone. I don't plan to take classes on an ongoing basis."
The other thing you can do, maybe, is if there is someone who lives with you (spouse, roommate, friend), you could ask them to watch you along with a video or book, then you can ask them to check certain aspects of your pose. I will sometimes ask my husband to notice certain aspects of a pose -- such as the flat, almost-curved back in a Downward Dog -- and ask if I look the same. My husband has studied martial arts and is very observant, so that has worked for me very well.
I hope you'll find something I've said helpful!! 