Drywall – Correct fixture for plasterboard that has another board behind it

Sometimes it's the simplest jobs that can be the most difficult. I'm willing to bet that its the summation of tiny errors leading to a visible flaw at the end. The question you want to ask is "How do I mark and drill a hole exactly where I want it to be?" (For a given value of exactly)

The answer to this question (and many others) is: Make a jig or paper template. Even if you're only doing it once, making a jig or a template is essential for precise work.

For this particular job, a paper template will probably do. Trace the outline of your hook (and holes) onto a sheet of paper. Now, at your leisure, using compass, straight-edge, GPS, and Lasers, find the center of the holes. Even if you just eyeball it, you're still more accurate than doing it on the wall, as you're working at a desk or table with good lighting and a comfortable working position.

Now, photocopy that sucker!

Cut it out, tape it to the wall, and start the drill hole with a small nail or even punch it with a nail set. You don't want the drill "walking" away from its start position. Go in with a small bit next. The idea is that the tool will do all the work so that you can concentrate on keeping that drill level, and on center. Go to your 1/4 inch bit for the plugs, and clean out the hole.

Remove paper, insert plugs, and you're golden.

It may seem like a lot of work, but the results are worth it.
1) Generally, you can't be accurate working vertically, so figure out a way to do the detail work on a flat surface. 2) Always start a drill hole with a nail or punch. It will always walk away when you power on the drill. (This is quadruply true on metal or hard plastics.)

You need to locate and identify the underlying structure, where and what. Only then can one determine the best attachment method and wall layout. Stairways typically have stringers under them running the length of the stairway run that you can connect to. But there are endless variations on stair construction, so only someone on site can determine this. There are various devices for locating underlying wood structure, their effectiveness varies by local conditions.

Stud framing is typically 40.5 cm spacing on center, so you may need two intermediate studs in addition to the end studs. You need solid blocking flat to the mounting face in order to mount plumbing fixtures. Some hardware needs to be framed into the wall, so you need to identify exactly what hardware is used in order to provide and locate any needed structure.

Unless you can provide all this information, no one can adequately tell what to do in enough detail to ensure proper construction.