There are a lot of variations of this out there on the wider internet, but here’s my super-simple version:
Take an empty plastic bottle, rinse if necessary, peel off any labels… Then poke some holes around the bottom half or so. I used an awl; whatever poky thing you use, be very careful not to slip & stab yourself.
Put your bottle in the middle of a plant pot, & start to fill it with dirt. (If you leave the cap on at this point, you will not accidentally fill the bottle with dirt.)
Your filled pot should leave just enough of the top of the bottle sticking out that you can remove the cap & fill it with water.
Plant and enjoy!