Heartbeast also has a video on the program "Tilesetter" (which has a free "lite" version). This can help you generate tile sets like he uses.
Here's that video: (www.youtube.com/watch?v=SI3mZ3ynrTw)
A few quick notes about tiles in Godot:
1) This configuration is only ONE way to do tiles. There are other formats Godot supports which can accomplish different effects (eg: animated tiles, randomized tiles [think of having 4 different "grass" tiles with slightly different images randomly placed to break up the repetition], etc.) You decide which tile configuration is best for your project (or your project's current layer).
2) Sadly, Godot doesn't have the most user friendly (or most powerful) tile mapping system. Thankfully, you can create tile map layers in other programs and import them into Godot. Good example: (www.youtube.com/watch?v=jFq4Eve_Db8)
3) Be aware that there is an issue in Godot's tilemaps where tiles may "flicker" on systems with Nvidia cards. You can fix this by sticking with GLES2, under Project > Project Settings > Rendering > Quality > 2D > enable both "Use Pixel Snap" and "use Nvidia Rect Flicker Workaround" by making sure the little check boxes are checked.