Don't mix it all at once. They will send you a small bottle (I think it's 15 ml) to mix in.
Normally if you mix a high mg liquid with 0 mg liquid you dilute the strength but you also dilute the flavor. Because the doublers are the same size as the
eliquid, you don't dilute the flavor.
Figure out how strong you want the final liquid to be. Then you decide how much of each to put in.
As you are starting with 48mg...
Equal amounts of each (1 part 48, 1 part 0) will make 24mg
3 parts
eliquid, 1 part doubler will make 36mg
1 part
eliquid, 3 parts doubler will make 12 mg
38% eliquid, 62% doubler will give you 18mg (I call it "a little over 1/3 eliquid and a little less than two-thirds doubler)