To make my plain glitter textures, I use paint.net's noise function to lay down a base texture on at least three layers, then quantize to limit the palette to a more recolourable state (usually 5 colours). I then use the ImAGif plugin to save them.
For the more detailed glitters, I've been using Tilemancer to generate tiling textures and then opening them as layers in paint.net and using the gradient map tool from Pyrochild's plugin set to recolour them. I've also been using a variety of the other tools from BoltBait's plugin pack for the glitter texts, namely the Outline Object, Grey to Alpha, and Apply Alpha Mask tools.
(note: Tilemancer is no longer being developed, but it's still pretty useable! Also note that when saving there's no filetype suffix so you have to open the files manually through the program, and you save the images by clicking on them in the nodes - remember to add .png after the name so you can actually open them!)