I know I'm not a boy... nor have floppy hair LOL. But I'll answer anyway
I never used to have a fringe and went to the hairdressers to get a side fringe put in (like what you want). It was done well, but when it got too long I attempted to do it myself - DON'T LOL.
You need to cut at the best angle and in a slopey motion. Thankfully I didn't mess my hair up, and went to the hairdressers and they did it.