Step no 1:
Harlequin effect is done using paint of only two shades. Start by painting wall with low and lighter sheen shade and dry it properly.
Step no 2:
If you want the diamond effect, you need to design the tape on the wall in a diamond shape, or you can cut the shape and sketch the design on the wall. The size of diamond shape can be determined by the length and width of your wall, whereas the common diamond shape height must be twice its width.
Diamonds must trace along with the width that can make the border of the room. Or else you can make one side of the wall as a central point with diamonds designed both vertically and horizontally.