Start with the right rope. Hollow braided soft nylon is relatively affordable and low maintenance. Have a smaller diameter (say shoestring size–You can use an actual shoestring if it's has little elasticity and is smooth textured), and a larger diameter one (maybe pinky finger size for average sized male hands).
I'd suggest using 3 double-column ties. If you don't know what that means, look up several examples–It's an elementary tie in bondage. Do not use single-column ties for connecting two limbs together. The 3 double column ties to use for this are:
- With a length of the larger diameter rope, do a double column tie just above the knees/lower-mid thighs, keeping the upper legs together. This is not strictly necessary, but limiting motion of the legs in general will further limit motion of the ankles.
- With another length of the larger diameter rope, do a double column tie at the ankles.
- With the smaller diameter rope, do a double-column tie around the two big toes. When doing toe ties, unless you really know what you are doing, don't get too creative and start tying other toes to things or to themselves. These can be very prone to injury.
You're probably going to want to tie another length of rope to the ankle tie, and then to a secure attachment point, such as the bed frame to keep the legs from moving around. They are very secured to themselves, but can still squirm around as a single unit.
One of the most important things for the double column ties is to ensure you can untie them easily and that they are not too tight. Beginners usually get these too tight, being very focused on the goal of immobility. During play, blood flow to the area can cause slight swelling of the tissue, which is usually unnoticeable, but causes the rope to fit tighter than when initially put on. Leave a bit of breathing room. Some say you should be able to wiggle a finger or two between the rope and the skin, but that's a guideline–Just be mindful and check in with them every now and then to ensure they don't have any numbness or tingling.
If you don't know how to tie an easy release knot (which, honestly for these, you could probably use a bow like for tying shoes), have some safety shears on hand. If you're new to tying, errors happen, and incorrectly ties can get tighter with struggling, causing dangerous limb constriction. Note: Safety shears are not scissors, knives, or rusty swords–Use the right thing for the right job to prevent serious injury.
Good luck and have fun!