How did the real Mohawk's get their hair to stand?

I’m guessing sugar water.

They were still using that greasy kid stuff.

~ ~ Maybe they had an exclusive contract with “DEP” https://en.wikipedia.org/wiki/Hair_gel

Great question!

From what I’ve seen in a movie the strip of hair on top of the head wasn’t that long so the natural rigidity of the hair seemed to keep it upright.

What I’d like to know is how did they shave the sides of their heads? Also how did they trim the top of the Mohawk to keep it from growing too long and flopping around?

They probably used some type of animal fat.
Hair could be carefully cut to length using a sharp basalt (volcanic glass) blade.
Hair could be plucked using fresh water clam shells.

I bet they pulled unwanted hear our, rather than shave

They didn’t.
Both sides of the head are shaven, leaving a strip of noticeably longer hair in the center. Historically the hair was plucked out rather than shaved
Also, don’t mix the hair style and the indigenous tribe name up.
Despite allegedly being named after the Mohawk people, an indigenous tribe that originally inhabited the Mohawk Valley in Upstate New York, history holds that the mohawk hairstyle we know today was more often worn by the Pawnee, who lived in present-day Nebraska and Northern Kansas

Maybe they shaved the sides of their heads and trimmed the top with very sharp obsidian blades. See here:

https://en.wikipedia.org/wiki/Macuahuitl

Its possible to get an obsidian blade to be sharper that a modern steel razor blade.
Or not. As @snowberry thinks, they could have simply yanked their unwanted hair out. But then again they would need something like forceps or tweezers to pull out short hair as it grew back.