Create a file like conventions.md in the root of your repository with specific commands for common tasks: running tests, linters, formatters, adding packages
Set this as part of the files it reads on startup. Then ask aider to look at your codebase and add to it :)
Aider has a lot of slash commands to familiarize yourself with. Ask and web are crucial commands to get the most out of it.
My recommendation to anyone is to use ask the most then tell it to “implement what we discussed” when it looks good.
Hope that helps