How can I stain furniture?

Is the furniture painted or stained already? If so you will need to sand it down. With stain you can brush it or they have spray stain now too that works pretty nicely. It is a Minwax product. Polyshades is the name of it and it is stain and poly all in one. If you brush it on, I like to use those foam brushes. Good luck.

Stain is easy (assuming the wood is raw). Brush it on, let it sit and wipe away the excess. When it's dry, lightly sand using 220 or higher sandpaper and apply another coat. Wood grain raises with the first coat, so that's why it needs to be sanded. When it's the color you want, paint on 2-3 coats of polycrylic, letting each coat dry between.

If you don’t wanna send it down, then you can strip it. Citra strip is the popular one

When I do staining with translucent penetrating stains, I like to use wood conditioner first to get an even streak-free finish. In a pinch, I will use a fine mist spray of water, wipe off, and then apply my stain. I use old soft cotton to apply and rub in quickly. If it needs more depth of color, I give a second coat.

Gel stains are more opaque and hide lots of flaws, they are more like paints IMHO. I use a foam brush for these or a small foam roller.

If you need to strip off an old stain first, be sure to buy a stripper that says on the front of the can it removes stain. Citri-strip won't do the job.