The higher the blade number, the shorter the clip. I've found my poodle can't really tolerate anything shorter than a #10 (though that may be partially due to my amateur grooming skills) on his face. For the body, I use between 6 and 13 mm blades (5 FC and 3 3/4 FC respectively), depending on how long he is and the weather. Some people prefer a #7 for a really short body (closer to 3mm), but I've always felt the 6mm was short enough for my tastes. You can get combs in these sizes too for cheaper - probably a better option if you're only doing this temporarily. I started with a set of different comb sizes, then later purchased blades in the sizes I found I used the most.
Also keep in mind that blades can get quite hot while you're working, which is a lot of what contributes to clipper burn. Keep an eye on the blade temp - if the metal feels too hot for your hand, it's too hot for your dog's skin. You can use cooling spray to help with this, or just be mindful and take breaks when it's getting warm.
And don't forget to oil your clippers after every use! Treat them right and they'll last much longer.
Again, I'm also an amateur home groomer, so there's a lot of things I don't know, and I've only really worked with the one poodle and our Lagotto puppy. Watching YouTube videos really helped me when I decided to do mostly home grooming, to get a feel for how to handle the clipper and tricky spots like paws and faces.
I have been grooming my mini-schnauzer for 3+ years. This is a monthly task due to the rate of hair growth.
First step is to deshedd, slicker brush and comb the dog. This removes any mats and large debris.
2nd, Bathe and dry the dog.
3rd, brush and comb again. This will remove any tangles from the bath/dry.
5th, toe nail trimming (weekly)
6th, tooth brushing (3X/week)
Don't worry. Its hair, mistakes will grow out. There are many techniques available on the Inet and YouTube.
Take your time clipping. Give the dog many breaks and treats. If the dog starts getting antsy or twitchy, then its break time. Remain calm. The dog will sense your energy.
The "classic" schnauzer cut on my dog is evident but not strictly followed. He does not have a beard. He had a beard, but I got tired of the constant mess in the beard and his drinking water dribbles. His mustache is cut at his bottom jaw line and the hair under the mustache close to the lips is scissored short, otherwise its in his teeth. I have not cut his body hair shorter than 3/8" (10 mm) simply because that is my shortest comb. His head and ears are trimmed without a clipper comb. His legs are rough cut with a 1/2" comb then finished with scissors combined with a steel comb. I allow minimum 3 hours to complete this task. Paws are rough trimmed with combless clipper then finished with scissors. There have been times when he became too twitchy, I just quit and finished the next day.
I have screwed up his cut more than once. Sure he looked a bit odd for a bit. Then the hair grew out and the error was soon gone. His eyebrows are never even or correct, but I do the best I can. He always moves just as the scissors are cutting........
Bears repeating. Its hair, it grows back. Don't worry.
One warning. Be very very careful with the ears and pads. If you nick an ear or pad, it will bleed profusely. Have some coagulant on hand. Septic powder or just plain baking flour or baking soda will work. Couple the coagulant with direct pressure. The bleeding will stop. Remain calm.