In Android studio, this can be done from the Quickstart panel, choose "Configure" then "SDK Manager". In the SDK Tools tab of the SDK Manager, make sure that the "Android Support Repository" is installed, and that the latest "Android SDK build-tools" are installed. Click "OK" to return to the Quickstart panel. You may also need to install API version 28 in the SDK platforms tab.
1. Open Android Studio. On a new installation, the Quickstart panel will appear. If you have open projects, close them using "File > Close Project" to see the Quickstart panel.
2. From the Quickstart panel, choose "Checkout from Version Control" then "git".
The bulk of the Session code can be found under src/org/thoughtcrime/securesms/loki on this repo and java/main/java/org/whispersystems/signalservice/loki on the session-android-service repo.
Code contributions should be sent via github as pull requests, from feature branches [as explained here](https://help.github.com/articles/using-pull-requests).