Basically you build Qt like you would for your normal host platform, i.e. you run configure and the make.
One difference is that you pass the -xplatform option to select which target you are building for.

As its argument it will need a mkspec for the target platform. There are several predefined ones, e.g. Rhapberry Pi, or you can create one for your platform if there isn't one yet.

You can google for tons of examples and tutorials for different common platforms.

Cheers,
_