To Multiclass in Microlite20 simply take an additional class at your next level break, subject to GM permission. It is not recommended for Games Masters to allow more than two class combinations however as this can lead to Powergaming Munchkinitis, a dreadful disease that afflicts some role-players.
Optionally, only allow a class change every third level (3,6,9,etc).

Certain class combinations can be used to simulate other base D&D classes:

Fighter + Cleric = Paladin
Rogue + Cleric = Bard

As an additional option, characters do not get the +3 bonus to the specialty skill of that class, only the first class a character takes receives this bonus. This should cut down on min/maxing and allow characters to branch into multiple classes freely.