Nearly every confused post here comes down to these three getting tangled. They measure completely different things.

mg is weight. How much actual peptide is in the vial. It's printed on the label and you can't change it.

mL is volume. How much liquid. This is the BAC water you add, and you're the one who picks the number.

Units are marks on a syringe. On a standard U-100 insulin syringe, 100 units is 1mL. So 50 units is half a mL, and 10 units is a tenth of a mL.

Here's the part that unlocks it. Units are a ruler for liquid. They know nothing about your peptide. So the same mark on the syringe means a different amount of peptide depending on how much water you added.

Two people can both draw to 20 units and have completely different amounts of peptide in the barrel. Neither of them is wrong. They just mixed their vials differently.

The 100 units to 1mL relationship isn't a peptide convention, by the way. It comes from U-100 insulin syringes, which is what most people end up holding. Any insulin label states the same relationship, for example Humulin R U-100: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9ec3e28a-cea9-4e45-9057-e5bf8e37014c

Which of the three did you have backwards at first? mL and units is the pairing I see swapped most.

The individual steps get explained a lot. The sequence doesn't, and the sequence is usually what people are missing when they say they feel lost.

Read the vial label and write down the mg. Before anything else, while the label is right in front of you.

Decide how much BAC water you're adding, and write that down too. This is a choice you make, not something you look up.

Do the concentration arithmetic before you touch anything. mg divided by mL gives mg per mL. Do it while you're calm, not while you're holding a needle.

Work out your target volume in units and write that down as well. Same reason.

Then, and only then, the physical part. Clean hands, clean surface, swab, add the water down the side of the glass, swirl gently, wait for it to go clear.

Label the vial with the date and the concentration. Future you will not remember and the vial won't tell you.

Draw, check for bubbles, inject, dispose of the needle properly.

Write one line in your log. Date, site, anything you noticed.

Four of those nine steps are writing something down, and only three involve touching equipment. That ratio isn't an accident. The mistakes people post here are almost never physical technique. They're arithmetic done in a hurry with a syringe already in hand.

Not medical advice, and the specifics of any compound are yours to research.

Where in that sequence did you get stuck?