Ah OK,
Unless you already have the boards perhaps a liquid (paint on type) of tanking system would be easier for you? Especially if the skimmed plaster is nice and flat. Do insure the plaster has been allowed its drying time too!
Else, most boards can be put up with 'dot & dab' method (do prime first). Dependant on the particular board used, some recommend a metal fixing then screwed through the blobs of addy when dry.
If in doubt, call the products technical help line to make sure the products installed correctly
Hope this helps