The marlinspike hitch is a great knot for making a rudimentary rope ladder. When tied around a pole or bar, it will resist movement in either direction, providing an ideal solution even for long term use.
Step 1: Prepare to Tie
Find a pole or bar to tie your rope around. Once you have practiced this knot, if you plan on making a ladder I would recommend using a thick dowel with holes drilled in it to slide the rope through to keep the hitch from slipping off the end.
Step 2: Tie the Marlinspike Hitch
Pay close attention to the photos. After forming the initial loop, make sure you don't form a plain overhand knot, as the rope doesn't pass through at that point. If creating a ladder, make sure you keep the ropes on both sides even, otherwise it will cause uneven stress on the rope and rungs.