Old floorboards are notorious for contracting over time, leaving gaps between the boards. It might be tempting to fill those gaps with wood putty or other material that dries hard, but you shouldn't. The floor is going to continue contracting and expanding with environmental changes, and that putty will soon be cracked and crumbling because it won't move with the wood. Hemp rope, on the other hand, will expand and contract in the same way the wood does.
Clean out the gaps between the boards, using your vacuum to pull all dirt and debris out of them.
Cut a piece of hemp rope to the length of each gap in the floor. For each one, unwind the layers of the rope until the width of it is bigger than the width of the gap by about half. (If a given gap is 1/2 inch wide, the rope should be 3/4 inch wide.)
Put on rubber gloves. Open and stir your can of stain. Immerse all the rope lengths in the stain for a minute. Pull them out and lay them on newspapers to dry. Let the sit on the newspaper for a full day.
Set a piece of stained rope on the appropriate gap, along its length, covering it completely. Press and pry the rope into the gap, using your putty knife. Continue along the length of the rope until it's sitting entirely in the gap and is flat with the floor.
Repeat the process with each piece of rope, with each gap that it goes to.