Did a round-trip to DFW and back a couple of weeks ago. Checked in online, and no bags to check, so just needed form from INM office. The person wasn't very verbose with instructions (as in, none) but we eventually figured out we needed to fill them out and give them back to her. I asked about filling in details. It wasn't a stupid question because the form, as printed, is designed for people traveling from somewhere else *TO* Mexico, not the other way around. We filled in names, passport numbers, etc., but as for the rest the lady indicated it wasn't necessary. No flight info, etc. She quickly scribbled "R Temporale" and "R Permanente" on the top of the form (in permanent marker) and then stamped the hell out of everything and we were on our way.
The people who collect half of the form are the airline personnel at the gate.
On our return INM collected the other half, which we had filled out w/ flight info, our address, etc., stamped our passports and we collected our one checked bag (we never leave with one, but always return with one for some reason) and we played red light/green light (the much safer version) and just like that we were at the taxi stand. Bienvenido!
One note of interest - when we were leaving DFW Airport for some reason there was evidently a flight bringing in several older Mexican couples. They had matching shirts, and as each one came out of the baggage area in wheelchairs a whole group of about 50 people applauded and took pictures and videos. People were crying and laughing and hugging. It was very nice to see.