How to tie a Overhand Knot
Make a crossing turn by taking the working end (the short end in the picture that does all the work) behind the standing part (the long part of the rope). Then bring the working end to the top of the knot.
Pass it throught the crossing turn and pull on both ends to tighten the knot.
How to tie a Double Overhand Knot
You can make a Double Overhand Knot by adding an extra turn. This will form a bigger knot. You can repeat this to make the knot as large as you want. Knots with many turns are called Blood Knots.
Form the beginnings of an Overhand Knot as shown. The tuck the working end through the turn again. Pull tight on both ends to tighten.
How to tie a Slipped Overhand Knot
A Slipped Overhand Knot is a useful stopper knot that can be untied very quickly.
Form a crossing turn as before. Form a bight with the working end (double it up and pinch it together to almost form a 'loop').
Take the bight into the crossing turn.
Pull on the bight and the standing end to tighten the knot. You can untie the knot by pulling on the short working end.