Stick the wafer (or the slide having wafer on it) to the stub, apply conductive coating and conducting path to the stub. That's all.

Dear Aleksandra Stankovic,

If you want to re use the wafer I would prefer Carbon coating, otherwise 30 sec gold coating would work for you.

Regards

Engin

Disclaimer: I've never worked with GaAs.

Since GaAs is a semiconductor, you may have success working with no conductive coating at all; try lower accelerating voltages like 5 kV and/or around 1 kV. If you see that coating is needed, use noble metals (Pt, Au/Pd) for better imaging or, if you need EDS, use carbon.