I assume you have already created the corrosponding ability data for an ability and configured it to what you require.
Basically, all you do is go to the UI button editor of the unit's data and add a new button (which is nothing more than art) to where you want it (usually bottom row). You then hook it to the ability you want (below where you choose the button more options appear). Then thats about it really.
Ofcourse there may be 1 or 2 minor things I missed out, however I am sure you will pick them up from existing abilites.