The plastic frame has to be removed from the old touchscreen, too. It's an adhesive. My new touchscreen didn't have one. When reattaching, insert the frame back into the DSi case first, then carefully drop the new touchscreen down on top of it to try to get it to line up correctly.
Also, after everything was done, I had to run the touchscreen calibration in the DSi menu to get the touchscreen to work properly.
I didn't get mine completely reinserted at first. The symptom I experienced when powering it back on is the power light comes on briefly and the bottom screen light flashes, then they both shut off. No flash from the upper display. Had to try reinserting 3 times before I had it in completely. I agree, DON'T REMOVE if you don't have to.
On Step 17, I only had the 3 black Philips screws on the left to remove.