While you can always download the extension straight from the VS Code marketplace, you can also optionally install it manually from the source code.
roblox-lua-autofills is built like any other VS Code extension. You can learn how to debug VS Code extensions with the official guide by Microsoft.
roblox-lua-autofills is slightly different from other extensions in that it has a "companion". The companion handles functions that are too tough to do with just plain TypeScript, and thus are written in Rust. While it's not necessary for a working roblox-lua-autofills, if you want a fully feature complete version, you will have to build it.
To build the companion, you'll first need to install Rust. From there, go into the companion directory and run the command cargo build --release. After it is done, you should be able to open up the target/release directory and see a built copy. Create a folder called bin at the root, and insert the release into there.
If you have any problems contributing, feel free to join the Roblox Open Source Discord and ping me (boyned), I check it regularly.