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.