There are many different models of Drain Guards most of which are held in place by the weight of the grating.
 
Pinching the fabric of the Drain Guard between the grating and the catch basin's opening is very effective. However, the excess material that is left is not always desirable. That material can be trimmed but for those who would like a clean/unnoticeable/trim-free installation, the Drain Guard, Economy Frame Model is a great solution! Added bonus? It stays put, even when the grating is removed.