A big part of making soap is being precise. You need to weigh out ingredients precisely so you have them in the right proportions. Never measure by volume.
First weigh the containers you will put the fat into, then weigh out the fat. I measured 1626 grams for the bowl and the fat, then subtracted 906 g for the Pyrex bowl, leaving 720 grams of fat. I also measured out 102 grams of olive oil in another cup and added that to the rest of the fat for a total of 822 grams of fat.
In order to use the right amount of lye and water, you need a table of saponification values. You can find one set here: http://www.tlcsoaps.com/oilsapval.htm
You can also use this online lye calculator: http://www.tlcsoaps.com/tlcsoapcalc.htm
Since I am aiming for 5% residual fat at the end, I used values of 0.132 for lard, 0.129 for olive oil, and 0.38 for measuring the water.
720 g lard x 0.132 = 95 g NaOH
102 g olive oil x 0.129 = 14.2 g NaOH
Total: 108.2 g NaOH
Now is a good time to put on your gloves and goggles. Weigh out the lye crystals into a bowl. It is OK to use plastic at this point because the crystals aren't wet.
822 g of total fat x 0.38 = 312.4 g water.
On this last measurement you don't need to be too accurate. Just add a little extra water if you're not sure, since it will mostly evaporate in the end. Put the water in a smaller glass bowl so you can mix the lye in. I used a coffee pot because it is made of glass and has a pour spout.
add the lye to the water. NEVER
add water to lye, because it can spit out of the container and burn you. Stir slowly as you add the lye, making sure to dissolve all the crystals. The water will heat up, but if you mix it slowly enough it won't heat up too much.