It's quite common to see artists and resin crafters mentioning they struggle with glitter sinking in resin, so here is a bit of information that might be helpful.
There is a big weight difference between plastic glitter and polyester glitter. Most craft glitter is plastic, which weighs more and will typically sink. Investing in good quality polyester glitter may help with having glitter suspend better in resin. Also, look for ultra or extra fine glitter which will suspend easier. Glass glitter will always sink, since it's made of glass, it will be the heaviest of glitters we have.
Most glitter can sink especially in lower viscosity resins which we often prefer for casting projects. To stop it from sinking to the bottom, you can add a ton of glitter so that it's almost paste/sand like. If you see space in your mix cup with clear resin, that will occur in your mould too, so fill the resin up with lots of glitter.
Another option is to mix 3 different densities of glitter together.
Many do use the glitter full paste mixture for glitter lines on resin geode art, but for some casting projects, that isn't always quite what we want.
Another option some artists choose, is you can wait until your resin starts thickening up, so the glitter will sit better in the resin, but you'll need to pour it quickly in to the mould at that point as well. If your cup is getting warm though, you will need to pour the resin in to the mould before it gets too hot in the cup.
The finer the glitter, the easier time you will have getting the timing right on getting resin thick enough to add the glitter, and have it suspend rather than sink to the bottom.
Please note, every resin brand is different. Some casting resins can sit longer to gel up, but some resins should not be left to sit in the mix cups (or it can go into pre-mature exotherm, overheat, gel and no longer be used, some call it flash-curing). So be sure you are using the right type of resin for your projects, and using it correctly per manufactures instructions.
Another option some artists choose, is to work in layers. It's more time consuming this way, but some state they are happier with the results this way. Have a tiny clear resin layer in the mould set first, then they put a glitter layer in the mould. Wait for that layer to get to gel point, and mix up a new batch and put some more in, and repeat. (but it can be tricky because the mixture has to be the same for each layer or it will be noticeable). So if going this route, you will need to measure. Also, be prepared if working in layers, and the mix isn't just right, it can all sink, then you may end up with a line on top with no glitter and everything else at the bottom. It can take time to get different methods right. Also remember not to pour higher layers than your particular resin product recommends.
Iridescent flakes are a nice option that don't always sink, and often will float nicely in some resins.. Some metallic flakes are also better at floating in different resins. Each product works differently in each resin, so you may need to experiment for the specific look you are going for.
There are some pretty mica powders that can look almost glitter-like in some pieces as well. These beautiful ultra fine sparkly mica powders will often suspend nicely in resins.