I love the Double RY4 too. I add AP, and also "Bavarian Cream" - the Diacetyl-free version.
Anyway - YES, making your own liquid is almost as much fun as trying out new hardware! LOL.
If you can make flavors you like, it's a huge $$ win.
My total cost runs about $0.055/ml, and that's NOT scrimping on materials...I get whatever I think will taste best. There are a TON of threads here to read through about mixing, but it is NOT difficult.
You need Nicotine, anywhere above where you vape, but most economical at 100 mgs/ml.
(I like Nude Nicotine, or Wizard Labs.)
You need PG and VG
(I recommend Essential Depot - direct or on Amazon).
You need some empty bottles. You need a way to measure - I use syringes.
You need some flavors - find some simple 1 or 2 flavor recipes that sound good and buy those flavors.
(I usually buy direct from The Flavor APprentice, but sometimes throw some in if buying nic from Wizard Labs)
I recommend a calculator -
(I like ejuicemeup, which runs on PC, has some built-in recipes, and lets you save yours)
Have fun!!