Figure out how much bacteriostatic water to add, and exactly where to draw to on an insulin syringe for your target dose.
Most peptides ship as a freeze-dried powder. Reconstituting means adding a liquid (almost always bacteriostatic water) back into the vial so the peptide can be measured and injected. How much water you add determines the concentration, which determines where you draw to on the syringe for a given dose.
Store the reconstituted vial in the refrigerator, out of direct light. Most peptides remain stable for 2–4 weeks this way (check the specific compound's page for its exact window) and should be discarded if the liquid turns cloudy or discolored.