So there's two "systems" for glamour, manual and plates. Manual glamour, you need the item you want to glamour in your inventory and a glamour prism. Then simply right click > glamour and you're done. Manual can be done basically anywhere, at any time.
Plates require you to put the item in the glamour dresser (which is at the inn and squadron room) and to be at the dresser to create the plate itself. Putting an item into the dresser takes 1 glamour prism. Once you've stored an item in the dresser you can make a glamour plate with it. Using plates don't cost any prisms, but plates can only be applied while in a city or instanced hub like idyllshire or rhaglr's reach. Plates are very handy if you have a full ensemble you like using and you can use plates to glamour same sets of gear by linking a plate to each gear set. So if you want to make your summoner and red mage have two different glamours, you can. Just keep in mind that the plates will not apply if you're out in the open world.