Kind of jumping in late to the party here but this is what I have done for DIY cream soda and I think it's pretty good but tastes do differ.
I mix everything up seperate in their own bottles and let them steep then put them together. I have found mixing flavors seperate is easier to get things tweeked to taste, plus I can experiment with different flavors as I get them.
I use flavors from Wizard Labs (TFA) and mix my e-liquid to 12mg of nic.
Sweet Cream Recipe/10ml bottle: 35/65 (PG/VG), 2ml of 60mg unflavored nic, 1ml PG, 6.5ml VG, 0.5ml of Sweet Cream(5%). Let steep for 24 hours
Cola Recipe/10ml Bottle: 70/30, 2ml Unf Nic, 4ml pg, 3ml vg, 1ml Cola(10%). Let steep for 48 hours
Champaigne Recipe/10ml Bottle: 65/35 2ml Ufl Nic, 4ml pg, 3.5ml VG, 0.5ml of Champagne(5%). Let steep for 48 hours. (Gives it that bubbly flavor)
I then mix to taste. For example in a 1.6ml carto I would put 0.2 ml of sweet cream, 0.2 ml of champagne, and the rest with cola. Let it sit for 20 minutes to mix together and vape.
Be warned I believe this to be a tank buster because it caused my carto to go cloudy. Could be because of the cola and/or champagne.