If your 25C-NBOMe is not in its freebase form (i.e. a salt) then plain water is fine to dissolve it. A ethanol/water-mix with about 20 % ethanol is better though, because that prevents microbial growth. This should last for quite a while (at least half a year without noticeable potency loss... perhaps way longer).
If it is the freebase, it would seem easy to just neutralize it with an acid (hydrochloric, acetic, citric acid...). This converts 25C-NBOMe to a salt thus increasing its water solubility. See above.
The dissolving process itself is very simple:
weigh substance with milligram scale, put it in a vial,
measure volume of solvent (syringe, graduated cylinder, ...) and add to vial, shake from time to time (be patient, could take a few hours!). This should yield you a clear solution (don't use suspensions, could be lethal). The concentration is mass of drug divided by volume of solvent for dilute solutions. Just to have some numbers: 19 mg NBOMe in 5.5 ml solvent -> 3.45 mg/ml. So 0.1 ml equates to about 350 micrograms. Of course you can adjust this to your amounts (don't miscalculate, could be lethal).
I wish you good luck working with such potent stuff
