(Apologies for the not-so-great-quality photos: the only digital camera I have is the one on my phone, and stupidly it doesn't have a zoom function.)
Step 1: Step 1
Step 2: Step 2
Step 4: Step 4
Step 5: Step 5
Your fold needs to leave a small triangle hanging over the edge (as shown in the second photo). This triangle will be used to seal the letter closed.