Step 1: Get yer stuff!
-military boots (or other boots that could be transformed into pirate boots)
-leather (sorry, I don't know how and where you can buy leather since I have a box full of leather sheet. I checked a bit on the internet and the price really varies depending of the leather quality and/or the quantity. But I'm squarely sure that won't cost an arm.)
-A sewing kit (thus including needles (you'll need BIG needles to sew leather, seriously I just broke one regular needle by doing this), thread (black thread fits well the leather than white/clear thread), scissors and a thimble (be careful if like me you're using a cheap plastic-made thimble, the back of the needle can perfectly well puncture the thimble and your thumb both while sewing leather))
And to attach the covering:
Step 2: Take meaayes (mesures)
-wrap around the top of your boot, the piece of leather that will change your boot's destiny
-then fold the leather in the shape you'll want to have on your pirate's costume
-mark where you'll have to sew the both ends of your leather sheet
-take off the covering of your boot.
Step 3: Sew!
Step 4: Sew! (part II)
Step 5: How t' attach this correctly
Well the covering won't stay on the boot by itself. You need a way to attach the leather on the boot, how about using the boot's buckles?
At first I planned to take a strip of leather and staple it to the inside of the covering, but because the leather is too hard or because my staples doesn't want to help me on my quest of cosplaying Guybrush or because I was using the crappiest stapler ever; I decided to use laces instead. It works too.
So with laces all you'll have to do is to tie the laces on the buckles like you would do with regular shoelaces, just think that a loose covering is easier to set but it wont stay as longer as a tight covering.