When you have 3 or more pieces, place your cursor on a piece that you want to transfer your mass to, then press W until desired.
It is most mass efficient with 4 pieces, though slower than with more pieces. Avoid 3 piece self feed unless absolutely necessary, it is a terrible waste of mass, and awfully slow.
Use self feeding sparingly, as a fraction of your mass will be lost from ejecting Ws.
A trick to get out of a bad situation (e.g. when surrounded by many bigger cells, or chased by a big enemy cell being pushed by teammates in Teams Mode) is to split away into about 8 pieces, then self feed. If the enemy isn’t as smart, they may multi-split into you, and you can eat them back. Use this sparingly as well, as it wastes mass, and contributes to triggering the Anti-Team Penalty.