You can start painting by switching to the texture or vertex color mode and clicking the paint button for the channel you want to paint on. A popup will come up which allows you to set a few basic values for the image you are about to create.
- Make Fully Transparent: This will make the created image fully transparent and not use the base color. It can be used if you want to paint over another image or color on the layers below.
- Base Color: This is the base color of the created image. The image will be filled with that color.
- Resolution: This is the resolution of the created image.
- 32 bit Float: This will create the image with 32 bit float enabled. You might want to use this for something like a normal map.
Click OK to start painting. Alternatively to switching to texture mode and then clicking paint, you can click on the brush icon which will skip these steps.
There are a few tools to assist you when painting. These are explained here: