I started with a pile of old t shirts in bright colors, washed and dried them,and used a rotary cutter to cut into strips about an inch wide and 4 inches long:
I cut up several shirts at a time, and sorted into bags by color. I used the mesh that is used for making latch hook rugs as the base:
And used a latch hook tool to pull one end of a strip of fabric through the mesh
And then tied a knot
Trial and error proved that I didn't need to put a knot in every row, I did two rows, and skipped a row, and skipped a row diagonally as well. the t shirt material will full up space and stand straight up, giving a nice thick, fluffy rug: