OneStopDIYShop and DIYFlavorShack have DIY stuff like PG, VG, flavorings, bottles, measuring stuff.
Use rubber or surgical gloves, eye protection, apron if you have one and do this away from kids and pets and clean up well.
The simple inexact way to make some juice. Mix a dilution with some of your nic juice and some PG. Figure out how much nic juice and how much PG will make 24mg nic juice or 16-18mg or 10-12mg or whatever you want.
I think approx
24mg = 5 parts PG with 1 part nic (your 150mg nic)
16mg = 8 parts PG with 1 part nic
12mg = 11 parts PG and 1 part nic
I do not know if your nic is in PG or VG. I use high PG juices but a bit of VG is nice in them - smooths it a bit and adds more vapor. My own juices are 80PG/20VG. You could replace small amount of the PG with VG (unless your nic is already in VG in which case you could just replace a bit of the PG in the lower nic mixes).
And then add flavoring. Obviously this is inexact - once you add flavoring you will be diluting the nicotine strength. Some flavorings only need 10%, some need 15%, some may need to be 20% of the juice (practice various amounts with plain PG to test what you want)
To do something ore complex and correct, here is an ejuice calculator
eJuice Me Up - e-Juice e-Liquid Recipe Calculator