We had an internally busted 3DS top screen resulting black portions of the screen like broken glass. Repair work was slow, taking our time like professional and all, most of the trouble was in rolling the ribbon cables like an ice cream cone. Taking apart and reassembling, that was the most difficult part. The rest is being careful, taking it very slow, reading instructions, and snapping it all back together. Not to mention FOLLOWING THE INSTRUCTIONS FROM IFIXIT.com! Doing those things should get you back in shape.
Slow, careful, neck ache. LOL.
Watch out for falling parts! There are definitely more parts falling out of the assembly unexpected, that were not mentioned in this tutorial. I can remember individual buttons on the d-pad falling out, diffusers falling out, just keep an eye on anything that appears loose, take it real easy. Have a clean workspace. Good luck.