You can do the same as shown here with round rubber legs. For hopper legs you can use three strands (un-separated), tie the knot (I use forceps pull through), lick the knot (absolutely mandatory to get the knot to stay put), tighten. Then you can cut off two of the three tag ends a the knot to make the legs appropriate size for the larger femur and smaller tibia.

For frog legs you a use four strands (unseparated) , make a knot, make another knot 1/2" away. Trim the tag ends to a length that looks good to you as a flipper foot.