This is a very nice, classic paper airplane. It usually flies very well, providing the folding is accurate. Enjoy.
Step 1: The Big Beginning
Get a piece of A4 paper, and fold it in half, length wise. Then fold the two corners into the middle.
Step 2: What Next?
Fold the tip down to the previous folds, like in the picture. Then fold the two new corners to that tip.
Step 3: Still With Me?
Flip it over and fold the triangle bit down. Then fold it back up again, but at a point about an inch down. Flip over.
Step 4: Fold, Fold, Fold Again
Fold one of the top edges of the triangly bit to the centre crease. Then sort of pull/slide the bit that goes over the centre crease back in line with it. Repeat on the other side.
Step 5: Hard Parts Over!
Fold the flaps into/under themselves.
Step 6: Finishing Touches
Fold in half, and then fold the wings, as in the pictures. Then add the winglets. Test fly it, and it it doesn't fly straight, cut trim tabs and fold as needed. Enjoy!