Are we talking about IV or IM injection? Just wondering, it doesn't matter a whole lot for measures and preparation...
Is it an extract? What color and form is it (Form as in crystals or powder or wax or goo, etc)? You really should NOT inject extracted DMT, if you don't know then assume it is extracted as is the majority of DMT.
Furthermore the reason it will not dissolve (not mix) in water is most likely that you have it in freebase form, which indeed do not dissolve in water (I probably shouldn't go into the chemical reasons unless you think you can follow).
The way to convert freebase DMT (if it truly is the type you can inject to begin with) is to add a correct amount of an acid. This acid should be a pure chemical like hydrochloric (muriatic) acid or fumaric acid, etc etc. You then need to calculate how much acid is necessary to convert the DMT (we can help you do that), add the acid to a little (but measured) bit of bacteriostatic saline and then add the DMT. The dissolved acid will change the freebase DMT to a dissolved salt form. Then slowly add enough saline to measure a total volume that both makes sense to inject dose per dose AND is enough to be able to contain all salt and DMT.
Then if you're done, run the liquid solution through a wheelfilter (IIRC it should be specified 0.2 micron pore) and into a sterile injection vial, or immediately disperse it over a number of syringes. Then store the syringes you will not be using immediately by packing them in properly sealed and sterilized bags or by vacuumsealing them. Put the vial or syringes in the fridge.