Well, if you want it to contain Nicotine, then you'll need some of the following (skip this step if you vape 0mg)
Unflavoured E-liquid Nicotine
Then you'll want bottles of plain old PG and VG to get the right percentages (if you're aiming for 50/50 etc)
PG based & Vg Based
Then you'll want some shit to mix it with (Syringes, bottles etc)
Syringes: (You'll need at the very least 10ml and 1ml syringes, 10ml for your Nicotine and other stuff, 1ml for flavour concentrates)
Terumo 1ml 2.5ml 5ml 10ml 20ml 50ml syringes 18G blunt mixing needle refill ink | eBay
Bottles: (squeezy with long thin tip seem quite popular)
Dropper Bottle Squeezable LDPE Childproof Cap Tip Liquid Ejuice Empty Wholesale | eBay
Or if you want UK based bottles, you can pay a premium here:
100 x 10ml child proof plastic squeezable dropper bottle THIN TIP - TRIANGLE CAP | eBay
Finally you'll want some flavourings, the base flavour of just about every custard seems to be Capella Vanilla Custard.
http://www.thealchemistscupboard.co.uk/product/capella
Then you'll want a mixing calculator, i use the POTV calculator on Android.