pretty simple, just download a Eliquid mixing calculator, there are a few out there, with the doublers for example you add your correct amount of nic juice to a separate bottle to get you to the proper MG then just add in your doubler to that bottle. This is a very basic explanation and it can get way more detailed when you DIY but the doublers make it quite simple, its like making a glass of ice tea after you get use to doing it for a while, just add x amount of doubler to Y amount of Nic juice all in your desired bottle size and shake shake shake and your done.