A tip: instead of using separate magnets for fixing your pictures you can make a nice frame from four pieces of a flexible magnetic strip – cut their ...
Step 2: Mark the area
Prepare the sketch of your magnetic surface. If you chose the wall near the stairs (like me) it will be something like this drawing.
Mark your area on the wall with a pencil. Use a spirit level. A tip (and one more use of adhesives): I don’t have a long spirit level, so I temporarily attached my level to a long ruler with double-sided adhesive tape.