ModPodge is just the brand name for thinned down permanent white glue (PVA glue) that's intended to be used for decoupaging (so it's referred to in that capacity as a "decoupage medium"). Other people just use a cheap permanent white glue (like Elmers GlueAll in the U.S.) and thin it with water themselves to do decoupage.
For screen printing and stencils, glues or kinds of "masking" materials can be used.
or other ways:
T shirt printing covers many parameters nowadays.
We are a screen print company who can print direct, however we can now transfer print, heat press and apply a flex material.
Other options available are embroidery and fairly new on the scene direct to garment inkjet printers.
It all depends on the quantity and complexity of the design, for anything over 50 shirts screen printing direct is still usually the best option.
Check out our t shirt printers site!