I've seen this trending all over Instagram & Pinterest. A simple and easy way to create a focus wall.
For my recreation of this D.I.Y I used:
Polka dot wall sticker kit from Rue 21
You could keep them as polka dots but I chose to cut them into triangles, they are safe for your painted walls and look great.
Dollarstore sells wall decals you can also cut to your liking.
Things to try:
Place them in rows, all over the wall randomly or even alternate sizes depending on the look you are going for.
Try other shapes! Another cool accent wall I have seen included little X's or + signs made out of black tape or wall stickers.