Hi Robin,
For this brand of resin, the manufacturer recommends mixing no more than 1 gallon total (resin plus hardener) at a time. Assuming you are starting with full bottles, mixing the full gallon of each is going to be too much at once. I would suggest pouring the desired amount into another plastic container, then warming that container in a warm water bath. When I use a water bath, I like to make sure the warm water comes to the same level (on the outside of the container) as the resin. You will need to be careful to make sure you don’t get any water contamination into either part, otherwise, the resin won’t cure.