So the first step is pretty simple. Cut off a working length about the length of your original shoelace. a longer working length might be a good idea. 1.2X the original shoelace length is probably a good place to start.
gutting paracord just involves pulling all that fuzzy stuff out of the casing and giving it to the cat. I didn't gut mine and regretted it. 15mil cord is too big to fit into the hooks of my boots.
After that, fuse your ends so they don't fray. Bonus points are given for fuses that are slightly larger than the body of the cord as these will keep the knots from coming accidentally untied.
Next, tie a bowline in one end of your working length with a nice small loop. a smaller loop looks cleaner and will allow you more range in how loose/tight your boots can be.
For how to tie a bowline, these guides are fantastic:
Lace starting at the topmost loop on the outside of the boot and continue to lace the rest of the loops normally.