Do both at the same time, this way you have a better fit.
Place a little around the rail being careful not to get any on the screw holes use a little wax on the screws and carefully place the rail on the action with a thin spacer between the rail in the middle and the action.
Try and start the screws then remove the shim and allow the rail to be tightened down. With Q tips wipe of all excess bedding compound on the outside.
I like to wax the receiver before I bed the base so I can remove it later and not leave any bedding compound on the receiver. (It all sticks to the rail for an easier clean up later on if you wan,t to use it on another action and it fits the action perfectly.