Make two diagonal folds, forming a cross as shown.
Make a third fold, intersecting the previous folds, but fold it the other way. (i.e. the first two folds should form a "valley" and the third fold should form a "mountain").
Pull the folds in to form a triangle (bit tricky until you know how to do it).
Fold the left and right corners of the triangle, so that they touch the top of the triangle. This should give you the frogs front legs.
Fold the straight edges of the paper inwards, so they meet (roughly) at the centre. This forms the body and legs of the frog.
Make two folds to form the legs. The top fold will form a "valley" and the lower fold a "mountain".
You should now have a frog (just turn him the right way up).