First off, don't stress it. You will be fine. I bought a drop weight YEARS ago (15?) and still use it. It is simple, accurate, pretty durable. I probably only average a racquet every 10 days... two of mine I restring pretty frequent, and two of my wife's which I do occasionally.
My first racquet took 2+ hours. Now, 45 minutes tops. I bought a Klippermate and the instruction book is one of the best instruction books I've ever gotten with any product. Don't know what you have, or if you have an instruction book. If you don't, you should probably find one on-line and print it out. It will help.
Simple mistakes are missed holes or missed weaves. No big deal. Use cheap string for your first job just incase you break the string. If you clamp the racquet properly and basically string the way you are supposed to, the racquet should be fine, unless the racquet was cracked already... but you should know that.
Make sure you have the proper string pattern. This may look like gibberish, but it is important. Which end to start on, 1 piece or two, where to tie off, shared holes, etc. Search for the string pattern for the exact racquet and write down the info... or type it here. Many of us have a stringer's "bible" and can look up a racquet easy enough.
After 3-4 jobs (if you are stringing the same racquet) you should start to get the hang of it. If you are stringing different racquets, there can be some big differences, so expect a wider learning curve.
What should you practice? Nothing. Just do it. Watch some videos for tying knots. If you think your knots aren't going to hold because of your inexperience, here are two things... #1- put a small dab of super glue on a knot to cinch it. Don't get any on the grommet or #2 - use a sharpie and mark the string close to the knot. This way you can monitor the knot to see if the string is slipping.