Billie: Here's a very simple one! ...what a great idea!
Here's one that uses vinyl to create a geometric look. It would be easy to repaint your wall a solid color and then add geometric shapes. They would be easily removed if you got tired of the look.
Frog tape works well to get even lines when painting.
Yes blue or green painters tape and make stencils by cutting a Manila folder with an Exacto knife.
Then pencil trace the stenciled layout and look at it for “balanced composition” before you paint the design.
There are some stencil sites for painting that may help, too. You could find a stencil template to use: https://www.royaldesignstudio.com/collections/modern-geometric-stencils
I use painter's tape and then go over the edge with satin water-based poly so it doesn't bled. If its a complicated design I draw it out or use a photo, and then use an overhead projector to beam it onto the wall and then use a pencil to trace it on.
You can also make a paper stencil and tape it on the wall.
I like to use mini rollers and foam brushes for applying the paint, if I'm doing a stencil I use a pouncing brush.
this is how I painted a geometric pattern on a door - it would work on a wall, too! https://www.charlestoncrafted.com/geometric-painted-door/
Before taping and painting, you should download a color photo of the wall and draw your proposed design onto the photo to see what the finished project will look like before diving right into the job. You may need to make adjustments.
Something like this? https://www.hometalk.com/44245242/geometric-accent-wall