Hi mat....
Firstly an expansion joint should go where there is already an expansion joint in the substrate...if there is one..like this pic..
these joints can be filled with silicone for light domestic use and you can use one the same colour as the grout...for high trafficked areas then a profiled expansion joints should be used..as in the pic...
These joints should go in about every 6 to 10 mtrs in either direction..this will reduce when tiling to heated floors.. these then become about every 4 to 6 mtrs in either direction....
If using the silicone method ( also known as a cold joint ) then just keep the joint clean and dry prior to using silicone....and if using the profile trim then it beds into the adhesive as shown in the pic.....hope this helps mate and good luck with the floor........