I started diy about 2 months ago. It's easier to do then most people think, with the proper reasearch beforehand. The things to keep in mind before starting are safety (especially when using nicotine), cleanliness (before, during, and after mixing), and storage (where and how to store your supplies, nicotine (if used), and finished product.
I started out using the weight method for mixing. I find it easier and less expensive, because fewer supplies are needed. I can mix directly into the glass bottles I use to fill my e-cig. If you use plastic bottles instead of glass, I recommend getting a funnel because of the smaller opening. Personally, I only use one syringe and that is for adding nicotine to my mix.
I've gotten my supplies from nicotine river and bull city flavors. Others may have different recommendations. I do recommend trying to get everything from one supplier to save on shipping costs. Also, some suppliers offer free shipping after you reach a certain price point.
As far as which flavors to get, I agree with what others have said, find a few recipes online with great reviews that appeal to you. Order the flavors needed to make those recipes. Make sure to use the same brand and flavor that the creator used. Otherwise, you will not get the same results. Taste is subjective, so start with buying 10ml bottles of flavorings. Once you discover which flavors/recipes you prefer and use a lot of, then buy larger quantities of those flavors.
Always label your finished product. You can use a label maker, or tape and a sharpie. It depends on how much you're willing to spend. Also, you'll want a notebook to write down recipes and your notes/opinions about the recipe. (Is it perfect the way it is, or is it missing something? Could a small tweek in percentages make it taste better to you?) Mix small batches to start with. If you love it, you can always mix up a larger batch. Taste is subjective.
I love diy! There is a bit of a learning curve, however, using proven recipies will help get you started. Also, some flavors are good as singles. Just be patient as you learn and enjoy the journey. It's great to be able to customize your e-juice to your personal tastes!
