I just started DIY a few weeks ago. There's a member named Hoosier out here who's quite the DIY wizard. Hunt down his blogs - also read the stickies at the top of the DIY section.
Mix in small batches at first (10mL or less) I find 10mL easy for proportions. There are a number of calculators
ejuice Me Up and Planet of the Vapes are a couple (I personally like
ejuice a little better - easier for me to figure out, but I could put PoV on my cell phone

)
Start with one or two flavor mixes at first, just to get the hang of it. There will be lots of recommendations on flavor percentages, but they're merely a guideline - your tastes will likely be different. If your flavors are too powerful, you can dilute with PG/VG. I'm not as hell-bent on an exact PG/VG ratio - I mix my flavors first and then top off with enough PG/VG to fill the rest of the bottle. I mix all my juices at 0 nic (my S.O. only vapes 0) and add a few drops of nic in my tanks when filling. It's usually not a problem for me to do it that way. I'm going to be traveling tomorrow, and I won't have as easy access to filling my tanks as I do at home or work. I'll preload enough tanks and carry one bottle of
juice with me (if I'm feeling spunky, I'll preload that bottle with some nic and top with one of my flavors.)
I ordered LorAnn, Capella, and FlavourArt flavors to start with. The Flavors Apprentice (TPA/TFA) is very popular as well. Check out
Electronic Cigarette, Clearomizer, USA e-Liquid, e-Cig Juice, DIY Flavors, Wholesale Nicotine, Empty Bottles for E-Liquid - ecigExpress - they had 15ml bottles of flavoring for just a few dollars each. There are lots of other flavor vendors out there - just look around for best combo of flavors and prices.
I've probably made about 15 different bottles, and only 2 have been questionable, but I'll vape them out anyway. It's a heck of a lot cheaper than buying. I love my B&M's flavors, but I just can't see spending $4 on a 10mL bottle when I can make one for less than $1. I've saved almost all my bottles from earlier this year. I have quite the collection of 6, 10, 15, and 30mls at the ready
Good Luck, and enjoy!