Use molding to mark the position and size  If it is Cement wall, we suggest fix plywood first (the plywood thickness: thicker than 9mm). And then fix molding on the plywood.
Mark the position and size on the wall Use tool to cut the angle and length you need, make sure you’ve cut longer than the size your measured
   
 Paint white glue between molding and wall  Tack the molding on the remark on the wall.
If you see gap at corner before you fit the other side, you could use hand jigsaw to fix the joint part of the molding If there is gap between molding and wall, you may use silicone to fill it up
After fix the molding on the wall, wipe out the white glue and silicone which was on the surface of the molding/wall Wait for the white glue and silicone dry, use sandpaper grind the surface and then paint it

1.  Use molding to mark the position and size.

2. If it is Cement wall, we suggest fix plywood first (the plywood thickness: thicker than 9mm). And then fix molding on the plywood.

3. Mark the position and size on the wall.

4. Use tool to cut the angle and length you need, make sure you’ve cut longer than the size your measured.

5. Paint white glue between molding and wall.

6. Tack the molding on the remark on the wall.

7. If you see gap at corner before you fit the other side, you could use hand jigsaw to fix the joint part of the molding.

8. If there is gap between molding and wall, you may use silicone to fill it up.

9. After fix the molding on the wall, wipe out the white glue and silicone which was on the surface of the molding/wall.

10. Wait for the white glue and silicone dry, use sandpaper grind the surface and then paint it.