I've been doing DIY liquid for about a year now.
1st you need to find a good supplier who has everything you personally want. I've been a big fan of My Freedom Smokes as my main DIY supplier.
2nd you need to decide based on your equipment what type of liquid you want to run. VG vs PG vs Mixes etc. My personaly method is to get PG based Nic and Flavors, fill with some PG and use VG as a final filler to end up with about 50% VG, but I'm using equipment that works well in VG.
3rd you'll need assorted mixing equipment. You can do beakers and cylinders for measuring and even pipettes but they can get messy. I just stick with 10 and 30ml bottles and use syringes to measure / squirt. A hot cup of water is all it takes to clean your gear after a mix. I personally reccomend having a couple of 5, one or two 10s and one 30ml syringe along with pierce top caps for your bottles of flavor/nic/pg/vg
4th you'll need math. I buy 60mg nic because I vape 6mg/12mg/15mg depending on my equipment so it divides nicely. My RBA hits like a truck with anything more than 6, but my eGo clearos work better for me on 12-15 depending on their ohmage.
For 6mg result -- 60mg / 6mg = 1/10 = 10%.
For 6mg in a 30ml bottle I put 3ml of my 60 into it for my end result.
For 15mg result -- 60mg/15mg = 1/4 = 25%.
For 15mg in a 30ml bottle I put 7.5ml of my 60 into it for my end result.
Same math applies to any str for any mix.
Starting Nic divided by Desired Nic = Divisor. 1/Divisor = %
5th you'll need recipes and here are two of my favorites. Start with Nic (Always) to desired strength.
My Mixing Method- First do Nic, then Flavor, then add PG to about the 50% line on bottle (I shoot for 50/50). Cap & shake like crazy. Add VG to top it off. Cap bottle, let it soak in hot water about 10-15minutes to get the VG viscosity down near the PG viscosity. Shake again. Works well in Clearomizers and Rebuildable Atomizers. Can do more or less pg/vg as desired.
Cool Minthol- Try 3% Menthol Flavoring (Premade), 1% Spearmint. Mix as above. This flavor can be vaped immediately.
Mellow Sweet- Try 10% Vanilla, 5% Amaretto (THIS FLAVOR IS NOT SAFE IN MOST PLASTIC TANKS). Mix as above. This flavor needs to sit a week to taste great.
Best of luck to you in your DIY, and hopefully you can find some nice syringes to use

The smaller blunts work best but with the pierce tops you'll want to pre-stab them with a sewing needle or safety pin that is clean. I personally use "Yorker Dispenser Caps" for my PG and VG fillers.