@Karyn I believe mixing the two separate is a better way to make sure the salt is dissolved. There's a number of discussions on the forum regarding if salt dissolves/stays dissolved in lye water. Because I wanted to make sure my salt for soleseife soaps is dissolved, I have chosen to do them separate.
I'm probably repeating a few things that have already been mentioned in this thread...
Salt saturation is 27%, so you will need to calculate how much salt to add that is less than 27%. I use 20% - and I also do not count the water that is needed to dissolve the lye. So... here's a standard recipe for me:
Total water needed is 344.81g, but 137.93g will be used to dissolve the lye. 344.81 - 137.93 = 206.88g
I want to do 20% salt saturation. 206.88 x .20 = 41.376g of salt
So now I can mix 137.93g lye + 137.93g water in one container, and 206.88g water + 41.37 g salt in another container. (hint: the salt dissolves faster if the water is slightly warm). Once everything is dissolved, I can add both to my oils.
This is how I do it - I've seen recipes on the internet that do 27% of the total water amount, which I think is incorrect as part of the water has already been used to dissolve the lye. Lye needs an equal amount of water to dissolve, so that's why I calculate my recipe that way. I'm sure there's a more sciency way to determine lye saturation, but I have found it easier to just mix lye water and salt water separately.