Just cut equal amounts off each stick, roll into a sausage and knead each one until soft and pliable, then plait them both together and roll, knead etc until mixed and a uniform colour.
NOTE:- start off with small amounts, smaller than you probably think you need, as it goes a long way. Press into gap/joint etc, wet with a finger tip and smooth over, and use an old scalpel blade to remove excess before it sets, smoothing remaider with a wet finger tip or wet blade.
Once set, it can be sanded, tapped, drilled etc and, before it sets, shapes can be moulded if desired.
Setting time depends on density/amount used, and can be between 1 hour and around 8-12 hours, when it should be rock hard.