This isn't really a fly, more a lure with a tied tail, but it has it's place in saltwater fishing as a good surface fly. After saying that, it's not a fly I use much as I prefer a fishing a Popper, but as someone has just asked me to tie him a batch of Crease Flies I can show you how to make them.
Start with a long-shanked hook, the longer the better, but in this example I'm using Kamasan B940 #1 hooks.
There isn't a pattern for these flies so you can use whatever colours and materials you like for the tail. I've tied a pinch of light blue flash under and some darker blue DNA type synthetic fibre over the flash as the tail, whipped it on securely and sealed the thread with a spot of superglue. The is the end of the "tying" part as the rest of the fly construction is modelling....
The next stage is to cut a piece of EVA foam sheet (I buy mine from "Hobbycraft") the length of the hook shank and deep enough so that when you fold it in half over the hook you can shape the outline of a fish body/head.
The shape of the body you cut is entirely up to you but aim for it tapering down to the tail but being fairly high and blunted at the head. The reason for this will be apparent later on in the instructions.
The next step is to superglue the foam to the hook. Fold it in half and glue along the bottom edge and in a diagonal from tail to bottom of head end. When you stick on the foam place it so that the top of the head end is still wide open. This acts like a Popper or Gurgler when the fly is retrieved.
The rest is now down to you and your artistic skills 
I've coloured the top half of the fly with a blue pen, added a few markings with red and black pens, stuck on some gold/red eyes then coated the whole head/body with a sparkly nail varnish.
Just to show you the difference between Crease Flies and Popper Flies I'm including a photo of a couple of poppers made using polystyrene foam heads.
