as i remember:
lay two long boards on the floor. lay two shorter boards on top of them, one at each end. you've created a rectangle.
drill thru both boards at each corner. use a bolt and wing nut there to tighten the boards after stretching the curtain. drill in several places along the short and long boards, in increments that you want. they will all work the same way. you will adjust simply by moving the boards.
from the back of the boards, nail through with nails long enough to go through the wood on the shorter boards and protrude about 3/4" - 1". on the longer boards, which will be recessed, you will need longer nails. measure so that they protrude enough to come up to level with the other nails. all of those are the nails you will work on.
NOTE: be sure that the wood is thick enough or can be trusted not to twist or warp as the wet curtain as drying.
can i ask a question? are you really going to dry curtains this way? if yes, please send pics. i haven't seen this in ten years.