Stretch fabric Boots
Two-Way vs Four-Way Stretch
First of all, get your mind out of the gutter. Second, stretch fabrics come in two fabric types: two-way and four-way. Two way means that if you tug a piece of square fabric in one direction it will stretch, but not if you pull the other sides. Four-way means it will stretch in both directions. This is important when making stretch boots because you need to ensure it will stretch around your shoe insert as well as your leg properly.
Four-way fabric is the best for stretch boot covers because it ensures it will stretch in all directions, both around your leg.
With two-way, you will need to ensure that fabric stretch is long ways around your leg instead of upward and downward. This means if you tug on the boots from the top they will not stretch, but if you it will if you pull at the back or sides around the thickness of your leg. If you have multiple panels you can play with this and put an opposing stretch around the knee in order to allow that flex if your boots go that high.
Ball Point/Microtex/Leather Needles??
Normally with our machines, newer cosplayers go for the default of 'universal needles.' Its the assumption that it is universal for the machine, but they're actually universal for fabric. It's also as universal for fabric as bike tires are for cars. 'Universal Needles' are generally cheaply made and the points give out really quick and, honestly, aren't the best for all fabric types. This is especially important when working with stretch fabrics. So look at the following:
Don your shoe and wrap your leg up tight
Personally I am a much bigger fan of plastic wrap/saran wrap, but plastic bags also work for this. Slip on your base shoe and make sure to cover every bit you plan on making a cover for: your shoe from base to top, your foot, your leg, your knee if you need to go that high. This base is essentially something for the tape to stick to.
With that done, take your tape tightly wind around your plastic cover. Start with some vertical straight pieces from the bottom to top to add some backbone support before twisting it round and round. With painters/masking tape, you're going to want to make sure you overlay the ends you tape so that they don't easily spread apart. The places you especially want to get is the base of your foot up to the heal, and then up the leg. Add some more vertical straight pieces when you are done to make sure the tape doesn't separate when you start spreading the pattern out.