If your attaching cloth to inside of a cloth bag, there's this stuff you can buy at a fabric store called stablizer. When heated with an iron, it will melt and essentially glue the two pieces of cloth together. It's used in collars and such to add stiffness. There are different types of stablizers, be sure to buy the one that melts and glues.
If your attaching cloth to a leather bag I would probably use hot glue. Just start in one corner of the fabric and add a few drops, squeeze together and move until the whole piece becomes completely glued. I use hot glue for attaching all kinds of stuff to leather. It's allways and instant bond and It's near impossible to remove the stuff after the glue sets. If you make a mistake you can often use a heat gun to remove if your careful. An old iron will smooth out bumps of glue. Just don't scald the cloth.
Regards
Loyd Shindelbower