for the best results grab a friend, sissors made for cutting hair, a comb with a pointy end, and a ruler if they arent so sure of their hair cutting skills..
start with the first inch (max) from the front of your hawk and trim it to the length you want then have your friend use the pointy end of the comb and gather the next 1/4 inch or so of your hawk. comb it up with the first trimmed section and trim all the hair to the same length. do the same to the next chunk of hawk except drop the first (not the second) section you cut before trimming the next (the reason for this is you want to keep the pieces your working with close together and able to stand straight up next to eachother otherwise when you continue trimming the length wont remain even). if your friend isnt sure they are cutting evenely they can use the ruler to measure the length of each section and keep on track. i know it sounds super confusing..im not great at explaining things, if i had fast internet id find you a video or something of what im talking about.
but the biggist tip i can give for this method is to work with the smallest sections possible for the best results, the more hair you gather together when trimming the more uneven it will be
one more thing, if your not so fond of the super straight just cut look cut with the scissors vertically instead of horizontally..it will give the ends a more natural grown look