How do you chart your cycle?

I have no idea how to track my cycles. I am so irregular that it's not fair; there are times when I go months without having a period. And then when I do have a period, I'm never sure if it's actually starting. I'll spot for a couple of days, then nothing happens for a couple more days, them it'll be really heavy for a few days only to go back to spotting. I think I'm done so I go out without a pad or anything, only...oops! Not done. Now I have to run home because I've messed up my favorite pair of jeans. Then it'll be heavy for the next week and a half to two weeks and then either stop suddenly or go back to spotting for a few days before eventually stopping all together. 

So, how in the world do I track that to narrow down when I'm ovulating? My husband and I have been trying for four ears and have only been pregnant once. It ended with a miscarriage at 7 weeks. My doctor just keeps telling me to keep trying, which is extremely frustrating because I've asked her to run tests if she has to. I am overweight, if that helps.