It is super simple. Most find it easiest to work with 50/50 which is equal parts lye to water by weight. The biggest trick is to make sure you stir extremely well and periodically while your masterbatch is cooling because you want to be sure it is all dissolved. I usually masterbatch 2 gallons at a time so I put my 3-gallon HDPE bucket in the sink in an ice batch because it will generate a lot of heat. Even when I first started master batching using a Rubbermaid pitcher, which btw works very well, I would put it in an ice bath so it would cool down faster.
When you are ready to soap you your recipe through a
soap calculator which will tell you how much lye and water/liquid you need. What you need to remember when using masterbatch is you have to double the amount of masterbatch if your recipe calls for 4 oz lye you use 8 oz masterbatch (this gives you 4 oz lye 4 oz water) because your masterbatch is 1/2 lye 1/2 water. You will deduct 4 oz from your liquid requirement and add in the additional required liquid. Many like this method because it makes it easier to use milks and other liquids without adding them directly to lye adding the additional liquids to the oils. . If you use our forums
Soapmaking Friend Calculator it will do the math for you if you check the box to use masterbatch.