What's the best method for sanding furniture to paint?

https://www.youtube.com/watch?v=LC2DDdhhkgY

Depends on how much varnish, sealer or paint is on it now. If your looking to get rid of several layers, then I would rent a sander, It's easy to operate, and you can rent one from a Homedepot or Lowe's. If it's scratched or marred, you could use regular sand paper, I don't know the grade you would use, ask in Homedepot or Lowe's.

Unless the furniture has a lot of flat areas, usually one ends up hand sanding the piece. With dressers and tables and whatnot with flat areas, a palm sander works really well and fast. Sand with the grain of the wood if known.

For intricate pieces, I'll use steel wool because I can get it more easily into areas the sandpaper won't go.

I start with a coarser grain sandpaper, 80-100 before graduating to the finer grains (higher numbers) for the finish coat.

The kind of wood, the piece, and the finish determine how much sanding I'll do. On pieces to be painted, I want it smooth which makes subsequent sandings easier. Bare wood will soak in paint and sometimes I get a little heavy handed with paint or I miss where it drips. Some paints require a sanding to create a "tooth" for the next layer. I usually sand in between coats, but it depends on how smooth the finish is or if the paint manufacturer calls for it. My end goal, even with painted furniture, is for the finish to be smooth to the touch.

Sand paper - you have more control..

I would use the finest grit possible to get the roughed-up surface you need to paint without destroying the grain of the wood.