To get these rough in dimensions make a sketch of the wall and note the following measurements.
Pedestal sink drain height.
Distance from the floor or wall to where the drain enters the wall or floor.
The drain stub out must be exactly behind the pedestal which hides the p trap and the supply stub outs should be high enough to be invisible without interfering with the sink.
Also matching the sink to the proper level on the wall and the exact height of the pedestal requires precise measurements.
Decide whether you can live without the storage space of a vanity cabinet.
Completed pipe work has to look neat.
Drain and supply pipes must be roughed in at the right height.
The drain line connects to the sink via a p trap a curved piece that prevents sewer gas from entering the bathroom and a tailpiece a straight piece that extends from the sink drain to the trap in most cases a standard tailpiece is the right length to reach from the sink to the trap but in the case of a high drain line or an unusually low sink the tailpiece can be cut to fit.
Before you purchase a pedestal sink remove the old sink and measure the location of the plumbing lines so that you can choose a new sink that matches up with them.