Two important questions:
1. How accurate is your scale?
2. How much material do you have total to work with?
If you want to measure 100mg of material, a 10mg (0.01g) scale is an absolute minimum. Ideally, you'd want to weigh out more material than that or use an even more accurate scale, but a 10% or so difference between actual and expected dose won't be a huge disaster with 4-aco-dmt.
Once you have a known amount of material measured at least reasonably accurately (rule of thumb is at least 10x as much material as the smallest unit on your scale, preferably much more than that but that should be good enough for 4-aco-dmt doses - so you wouldn't want to try to measure less than 1.0g if you're stuck with a 0.1g scale), you will take a known volume of water or alcohol (I believe 4-aco-dmt is soluble in both but double check this first) and dissolve the material in it. Once the material is fully dissolved, simply divide the amount of material by the volume of solvent (so 100mg of material in 50mL in solvent would be 2mg per 1mL) and use a syringe to measure out the appropriate volume for your desired dose.
Do you only have 100mg total material, or will you be weighing that out of a larger amount? Don't take your supplier's word for it on how much material you have - people sometimes get half or double what they ordered, which can radically throw off your dosing! If you have at least a gram of material, you could just weigh the whole amount to verify that you have the amount you think you have and dissolve it all into solvent. You'll want to store the leftovers in the freezer, either in a dark bottle or wrapped up in a bag or something to keep it in the dark.