The most common use for fabric glue is patching holes and rips in textiles. It saturates between the fibers and leaves behind a tacky film without soaking all the way through. Like rubber cement, you want to coat both bonding surfaces with adhesive before they are mated.
To demonstrate this principle, I am fixing my bud's pants that had an unfortunate tear in the rear after getting snagged on something sharp. He tried to fix it once with stitches, but because the denim frays so easily, the stitches tore out. Perfect reason to glue in a patch!
For any patch you will need:
Turn the garment inside out and slide a piece of cardboard behind the tear. If you are working with delicate materials that are prone to moving around, use some masking tape to hold the rigid backer in place.
Using tailor's chalk to outline the areas where glue needs to be applied is a good non-permanent way to make marks on fabrics.
Use a brush to coat the surface areas that are going to be mated. I like using a fan brush with these globby glues because it kind of has the same feel a giant cotton swab may have, but doesn't leave behind any fibers.
Mate the patch with the torn fabric and allow to cure for the time stated on the back of the glue bottle. There are lots of options for fabric glue, so make sure that both your patch and your glue can flex and bend like the material you are patching.
If you need to smooth out your glue lines, use a burnishing tool or another piece of square cardboard. Excess adhesive may cause bunching in the repair.