Yes, you should. And I have tips. Get scissors, or clippers (you can buy expencive ones at walmart, but its worth it esspecially if you plan to keep cutting it at home), chair, spray bottle, hair comb, and a rain coat. Have all these things ready before you tell her you are cutting her hair, while she is at school is a good time to set up. Have her sit in the chair. If you are using clippers, plug them into the wall, put 5th blade on, have her look down, and run the clippers up her head to the middle of her head, and then top and meet it at middle. (it may help if you cut her hair to her neckline, with the scissors, first.) this will give her a very short haircut, perfect for summer! If you choose to use scissors, wet her hair with spray bottle, brush hair with comb, and snip it to desired length (or, put it in a pony tail and cut it off that way.) if you plan to just snip it, hold one shaft of hair between your index and middle finger and pull it down to desired length (I say neckline, its also great for summer.) this will give her a short and sweet haircut. 12-year-olds are sensitive about the way they look, so don't talk to her to much durring the cut, unless she willingly brings it up. Like one time, I was at my aunts house, and she was cutting my cousins hair, and that was all aunt could talk about, so, my cousin made her sthp and had ME cut 10" off her hair. Ever sence then, my cousin has had me cut her hair. She grows her hair out durring the year, and has me cut 5" off before summer, as her hair grows fast.
Hope this helped