Gel material is available from camera stores, but you could use any coloured acetate for this. Get adhesive-backed velcro strips from a fabric store. If you haven’t already got velcro attached to your flash (for mounting commercially made foldable softboxes for instance), then make sure you buy both the ‘male’ and ‘female’ halves of the velcro system. One gender goes on top and bottom of the flash head, and the other in two strips on each gel square.
For my flash, I cut the gel into 3.5″ squares. This size has the benefit of fitting into a mini calendar jewel case (two gels fit into each case). My cases came from Effectuality (their part # CCM0CT). If your flash head needs more coverage than this size provides, these may not be big enough.
Just attach a gel and fire away. I like using the flash off-camera in optical slave mode so that I can use it to highlight a small area in a particular colour for a bit of drama. In the twin-lens camera shot below, my main strobe was used with an orange gel, and the flash with a blue filter off to the right provided the blue highlight.