Absolutely, you can do it yourself. At first, you need a mirror and a metri ruller. Relatively, with the help of your families, what you need to do is just to stand still there. The distance between the centers of your pupils is called pupillary distance and its typical range is between 57 and 65 mm. Second, look straight ahead and keep your shoulder relaxed. Now, one of your families can hold the ruler in front of your eyes, parallel to the floor, estimating the figure. Keep it in mind that your pupilary distance changes as the movement of your pupils. I mean, when you look close to read, your eyes will converge an average of three millimeters. So, it is important to ensure your eye level and focus on one point stably.
Ok, from what you said, I know you just want to figure out your pupillary distance data by yourself. However, I mean, we have a lack of experience and necessary equipment, which makes it difficult and not exact. So, you'd better head to an optical store and ask for that service, shouldn't be much too pricey. For your safety concern, just get it done by a professional.
I do not suggest you do the measure of the pupillary distance by yourself or your family members at home which may have the mistake. You could go to the real store of the eyeglasses to do the eye exam which will give you the exact data of your pupillary distance given by the scientific machine. Then you could base on this data to choose the suitable eyeglasses from the online store.