Or pretty much any open source project on github.
Start with downloading and compiling LND from scratch, if you haven't already done so. A Linux system is probably easiest to get it to work on, but I have heard of people who run it on Windows too.
https://github.com/lightningnetwork/lnd/blob/master/docs/INSTALL.md is the procedure you are looking for.
Run this towards the testnet. Upgrade daily. Become familiar with git commands to pull changes, compile and install LND.
Now, start with creating a github user if you don't already have one. Read the code, the structure of the project, understand how issues are handled and PRs are handled - in an organizational way.
Don't worry at first if you don't think you have the minimum coding skill sets. There's still documentation to be written, fixed and clarified, and reading code will make you understand things better. Small bugs can be fixed even if you're not that skilled coder.
Fork the LND project, to your own user! See https://guides.github.com/activities/forking/
Now, replace the working copy in the daily compile/upgrade workflow with your own project! Download and compile it, install and upgrade your testnet node with this.
Next step is to get changes from the main LND project incorporated into your own fork on a regular basis. You can follow the guide here: https://help.github.com/articles/configuring-a-remote-for-a-fork/
To keep your fork up to date, you can follow this guide: https://help.github.com/articles/syncing-a-fork/
To push this to your fork on github, you'll do
git push origin master
It is extremely important to use this fetch/merge method and not doing git pull from the upstream repository.
Now, work with this scheme for a couple of days, become familiar with the fetch/merge routine and pushing to your own repository.
Now you are ready to create your own changes in your fork. You'll always want to keep the master branch free from your own changes and continuously synced with the upstream project.