This is a great question.
You should start by calculating the intended dosage per serving. A good rule of thumb would be to aim at 10mg of THC per serving. This way, people can self-dose by taking half, or two dosages according to their known preference.
Once you have your intended target dosage, you need to make sure that the cannabis infusion you use has an accurate strength in terms of THC content, and that you use the right amount of infusion to create your desired dosage.
So for example, if you are creating brownies, and you intend to slice your tray into 8 pieces of 10mg each, you will want to add exactly 80mg of Cannabutter to the brownie mix.
If you are making your own Cannabutter, or cannabis infused oil, you need to pay careful attention to the recipe to make sure you get this right. Even then there may be some variance.
Finally, I would suggest you try the final product on yourself first before offering it to others. Make sure that it feels about right, in terms of THC content before sharing it and potentially overexposing others to THC. Edibles are notorious for giving people bad experiences from overdosing due partly to the reasons above, and partly because they take a long time to kick in and a long time to go away. Too much THC can be a very unpleasant experience to the novice consumer.
Make sure you mark your final edible product with some indication of the expected THC content per serving, and make sure you keep it away from pet, children and others who may eat it without knowing that its made with cannabis.