How did you learn to sell?

If the job-to-be-done for the customer is well understood during product development, then I’ve found that the product sells itself. Your job is just to look at the world through the customer’s eyes when building your messaging. For instance, I love this line from the book Product Led Growth by Wes Bush (Chapter 8). “Often, it’s easy to think that we sell products based on the functional outcome they accomplish. Businesses need live-chat software, right? No. Businesses do not need live-chat software. But, businesses do need a new and better way to acquire customers. The difference is that we’re selling an outcome.”

Just do it, that's my opinion. Reading some relevant books can also boost your selling skills, but the best way to learn is by doing it. You might suck at first, but the more you do it and improve, the better you will get. Good luck!

I was a junior sales representative for a Microsoft partner and I became really confident in my skills after that experience. Doing that for a few months has impacted my sales skills a ton

By doing it! Build a small product (timebox the dev time to a few weeks max), stop the product development, and try to sell it. Again, timebox your efforts, and try to make a few sales. You will learn fast what works, what resources you need, what acquisition channel is the best, etc… You can read thousands of books about how to sell, at the end of the day you need to do it to really learn!