Homemade powdered jello recipe
Some quick digging brought up a recipe for gluten-free jello that used a ratio of :


1TB gelatin
3-5TB granulated sugar
2c. fruit juice


There are lots of similar ones out there ... some call for more gelatin if they're planning on putting it in a mold (1TB/cup) or making squares you can hold (1.5TB/cup), and some replace part of the juice with a fruit puree.

The exact amount of sugar is going to depend on how sweet the flavoring is, so you might need to play with this one a bit to experiment.  And you may need some citric acid (or a citrus fruit powder) to balance out overly sweet flavorings.

I would also be careful about both how I stored and hydrated the mixture.  Sugar will absorb moisture out of the air, which might result in the mix clumping together before you can use it.  I'd try to put it into pre-measured containers that have little headroom, or where the air can be removed (eg, zip-top bags).

For hydrating, you'd want to mix it into maybe 1/8 of the total liquid and let it sit to hydrate.  If you see any powdery lumps, break those up before you add any hot water.  You could then add 3/8 of the water as boiling and stir to dissolve it completely, then add the remaining 1/2 as cold water to cool it down.

I'm sorry I couldn't be more specific about this.  My attempts at making gelatin used liquid flavorings.