The wall is only just deep enough to conceal the 32mm basin waste (solvent weld rather than push fit)
Cut the plaster away from the bathroom side of the wall or sometimes its better to cut it away from the other side which may be a bedroom or airing cupboard. Locate the position of any electrical cables before cutting the plaster board this can be done with a cable finder.
Cut the plaster back until you find timber or metal studs.
This may end up being a large section of wall.
The services or pipes will come from the loft or the floor or possibly both.
The waste needs to go out under the floor or be boxed in above floor level.
The top fixing point is the basin fixing holes. This should have the largest possible section timber fixed at the correct height.
The next fix needed is the semi ped if it is not hung from the basin with spring clips but is screwed to the wall. This fix does not have the load of the basin so a section of 9mm ply will be strong enough which is also fixed to both studs.
This ply will probably need to be cut in the centre to allow the waste to pass it.
The waste plus the hot & cold supplies need to be tested & clipped before the plaster is reinstated. Also before the plaster board is replaced take a long straight edge like a spirit level & mark the old original plaster which was not disturbed with the fixing points so you don’t move the position of the basin & screw through a pipe.
The pipes coming from the wall need to be long enough to penetrate the plaster any tiles or panels the reach there termination points which for the waste is the trap & for the hot & cold is the tap connection.
Cold on the right & hot on the left.
Yes it is a fair bit of work