when i did it i lined up the two long edges of the bottom and sewed them first. then, i took one of the short sides and sewed the two pieces straight together, and they formed a sort of pinched look that made it stand up. i'm not sure what the right terminology is. i did it that way as well as the tutorial way, and they both came out the same. i hope this helps? the way i put it sounds confusing.
i think that sort of helps! i did what you did (sewed the two long edges of the bottom first). after that, is when i sort of got stuck because when i sewed up one of the short sides, it got attached to the lining because the back of the lining got sewed in as well.
i'll keep trying and use your hints