Get a double-sided dresser, which are long with two columns of drawers. These are generally the right height and width for a desk. Decide which side you want to use for your seating area. You will be hollowing out the bottom area of that side to make room for your chair and legs.
Remove all the drawers and set them aside. Remove all the drawer tracks and hardware on the side you are hollowing out.
Hollow out one side of the dresser for your legs and chair. Remove the inner framework from that drawer column with a jig saw or hand saw. Cut away all of the framework that lies between the center support to the outer wall of that drawer column. Leave the framework in the drawer column on the other side of the dresser intact so you can return the drawers.
Sand the inside of the desk on the areas that you cut to smooth them out with a medium grit sandpaper.
Look at the support that divides the two drawer columns. If it is a solid wall, you don't need to do anything to it. If it is open framework, measure the height and the depth of that inside wall.
Get a piece of chipboard cut to the measurements you took. Cover the space with the chipboard and nail it to the frame with tack nails to secure it.
Replace the drawers on the side that still has the drawer tracks.