Expansion is the ability to take control of additional mineral lines which you started the game with. The major or primary goal will always be to have your major mineral line saturated. This is when you are still at the starting point of the game. However, once you have succeeded in doing such, you may have to start considering how you can have two mineral lines saturated. Players can control two or more mineral lines.
It is important that at the early stages of the game, you should have hatchery, nexus, or command center built. At such point, ensure that over 50% of resources gatherers which you have is transferred to your expansion. This will make the same number to be present in your former and new mineral lines. The timing of any expansion isn’t important in any way. Rather, what is actually important is the fact that you can build about two groups of gatherers at once.
The same way you practiced 1 command building, that is why one gatherer will be built at each command building (in some few seconds). Continue doing this, until both of the buildings are containing twenty four (24) workers and additional six for geysers.