Contributing to TEdit
To modify TEdit source you will need a few things:
Note: VSCode does not support modifying the UI

Fork and clone the repository

See GitHub Documents for the current method of forking the TEdit repository https://github.com/TEdit/Terraria-Map-Editor.git.
Use git to clone the repository to your local machine. Pick a folder and execute the following command: git clone https://github.com/TEdit/Terraria-Map-Editor.git.
You can use ssh or git extensions if you have those set up. For for detailed instructions see GitHub Cloning.

Open with Visual Studio

Open the folder you cloned TEdit source code, navigate to the src folder and double click TEdit.sln

Making a change

Check out these tutorials from Microsoft:

Commit to GitHub

Follow the instructions here for how to submit your changes back to your fork of TEdit on Github: Committing Changes

Submit a Pull Request

Finally, submit a pull request back to the master TEdit repo: Submit a Pull Request

Localization and Languages

Please see this PR for an example of the edits needed to add your own language: https://github.com/TEdit/Terraria-Map-Editor/pull/1391