Fitting wall units onto wall

Hi

I have just bought a very old stone house.

Doing a lot of the work ourselves and wanting to put the wall units onto the wall.

The wall has been knocked back to the stone and had plasterboard dot and dabbed on and then skimmed.

How do we attach the wall units on safely so that they hang properly and do not fall off?

Need step by step advice as very new to this and do not have enough money to pay somebody to hang them for me?

Many thanks