How to make this dress shorter without a professional?

URL got truncated, so this is my best guess of what you're working with.

Figure out exactly how much needs to come off the hem. If you think it's three inches, find something solid and 4 or 5 inches thick to stand on. Have someone measure exactly how much needs to come off.

Now here's where it gets tricky: when you cut off 3", the hemline is likely to pouf out a smidge because the weight of the rest of the fabric isn't pulling on it. Given three layers of tulle, I'd pull the first two up out of the way, and make myself a template of the exact amount that needs to come off, minus 1/2", and I would carefully mark the innermost layer of tulle at that length and cut off the distance that I'd marked. Then watch what that layer does.... it *should* be half an inch long, but it's probably just 1/4" long. and it might be just right. Figure out if extra needs to come off that layer, and do it. Double check. Once you've decided on the exact amount to trim, you can make a template and mark the outer layers of tulle and cut them off at the correct length, *one layer at a time*. Since tulle doesn't ravel, that's all you need do there.

The lining will also have to be pinned up. Since you probably don't want a 3" deep hem (but you might!), you'll need to make yet another template , one hem depth + 3/8" less than the template you made for the tulle. (Let's say you actually needed to remove 3.5" of tulle in the final analysis, and you want a 1" deep hem... make a template 3.5-1.375" = 2 1/8" wide. That's the amount to cut off the lining, then turn the hem up and hem by hand or machine.

Don't try to cut tulle "by eye" -- it never comes out straight that way. Take the time to do the template and draw and cut each layer individually, and it'll look professional.