What is the best way for an intermediate programmer to gain experience? The jobs I had before gave little to no feedback on my work other than was it done on time and does it work, I'm not confident enough in my knowledge to contribute to open source and I feel like I need guidance on best practices and such. Any suggestions are welcome.

Just practice programming as much as you can. Find some easy ideas you can realize alone.

If you need guidance, find some open source projects with a nice community. You don't have to be afraid of them, most of the people are happy to give you good advice. Just don't take criticism too personally. Working with other people helped me the most. Maybe you find a nice collab on devRant :]

It depends on what language you are using. Most languages have best practice guides.