It is best that you have some kind of scale, at least so that you're sure of the amount of material you are starting with. Now the most accurate way to dose is liquid measure. There is an in-depth guide about this method here.
First you need to make sure you know the weight of the material you are starting with. At least weigh it on a scale with a readibility of .1g to see if it is relatively what your source claims it to be. Then choose a solvent, bottled water should work fine but perhaps ethanol would be better if available. As opposed to getting a bottle with dropper it would be easier to get yourself a syringe. Many grocery stores have baby sections and you will likely find there an 'oral syringe' with measurements in the ml. Get a beaker or something that is also marked so you know precisely how much liquid in ml you are using. You can check the integrity of your utensils by pouring x amount of liquid into your beaker and making sure the syringe measures the same as the beaker. Let's say you have ~250mg of 2C-I, you could dissolve it in 62.5ml of distilled water to get 4mg of 2C-I per ml. Carefully push out 5ml and you have a dosage of ~20mg.
I just remembered that some droppers conveniently have measurements in mililiter increments, especially doppers used for administering infants tylenol (ironically). Anyways that is basically what you do but I've never worked with 2C-I so you might want to search Bluelight for more advice about solvents and such.