Most people doing scoubidou use this knot. Its name is crown knot, but is has more names.
Tie four strands together or start with the loop I explain later.
Spread the strands as the spoke of a wheel. You can hold the start with your first finger. Cross the first string over the second, keeping a loop at the arrow. Cross the second strand over number 3, (doing so it also passes over strand 1.) Number 3 over number 4. The last goes through the loop made with strand 1.
Pull nearly tight, than really tight.

Keep making this knot and your string will become round. That is why sometimes they call this knot the round knot. If you want the square knot you will have to make each second knot the other way round. The knot itself is the same, just mirror image.

A loop to start your work with can be made by folding two strands together and than tie a knot in the lot, it works but does not look good.
If you just cross your strings and than tie a first knot, pull not tight and pull the other string at the back into a loop. Tie a second knot and the loop is fixed.
At the end of the work you want your strands to stay in place. With the plastic strands sold for scoubidou you only have to pull the last knot extra tight. Than cut the ends short. Sometimes you have to help a bit, warm the strands a little, often holding them in your hand will be enough, pull the knot extra tight and leave overnight. If the knot is still tight the next day it will stay tight.

If you work with string, or if for some reason your scoubidou string does knot stay in place, you can make an other knot. Make one more knot but do not pull tight, tuck each string through once more. In the drawing I have made little spots in its own 'color', go underneath the find it.

If you have tied the strands this way and the still come undone, search internet for another knot, or use a little glue. Test your string with the glue, plastic and super glue do not always go together well. .

HELP!!!

I run out of string but I am not ready by a long way.

Place a new string along your little end and tie two knots with two strands together. It shows, you can hide it a little by making the ends thinner, just pull on the plastic.

Have fun.