The hardest part is to come up with the idea, but it can be difficult to get it to look exactly how you want it to as well. It's much easier to copy something that already exists, if you see what I mean. For example, with my Olympics sigs I was heavily basing the idea on the header at the GB Olympic website. So, yeah, the idea is the hardest bit, but it can be very hard to get it to look just as it does in your head.