I haven't bought any premix since about 3 months after I started on e-cigs... Jumped into DIY with both feet and haven't looked back yet... Took me a while to get the hang of tobacco recipes (the hardest I think to get a handle on), but the others haven't been much of a problem...
If you're hesitant to trust your own math just get a decent e-juice calculator (like EJuiceMeUp) that'll do the math for you... Pick up a nice selection of syringes and a couple of smallish beaker, some gloves and goggles for working with the nic, a few flavors, and you're pretty much good to go... Check out some recipes found here or elsewhere on the Web before you order your flavors and target your flavor purchases around those recipes...
Still, though, one of the best ways to get a handle on flavors is to start off making single flavor mixes... That way you can start to determine your preferred flavor % for each one and save you some problems along the way...
But the best advice of all is, as has been stated here before, if you think you're interested in DIY just go for it! It's not all that expensive and, if you follow through, you'll save a ton of money in the long run and have a lot of fun creating your own recipes along the way...
