[1.15] TileEntityRenderer guide

[Villfuk02]

So I have implemented and registered a TileEntity with TileEntityRenderer but i have no idea how to use it. The vanilla code not being deobfuscated doesn't help either. I just want to render a transparent quad with a tinted texture - supposed to look like water or another liquid. Where do I start? 

[Villfuk02]

I've figured out how to render textured cuboids, but how do I make them transparent and how do I tint the texture? (It doesn't seem rendering a quad is possible, only cuboids)