I prefer the poi made with ropes as opposed to the ones made with nylons because the nylons wear out faster than the ropes do.
Step 1: Cutting the tennis ball
Step 2: The rope and washer
Step 3: Assembling the poi
Step 4: Final step
because of a very good point made in a comment, i am posting a pic of the knot i use to tie them on to my hand. after copying the pic, then simply pull the knot fairly tight, slip on to your hand, and pull tight.