http://www.thesage.com/calcs/lyecalc2.php is the addy for the calculator, it is a pretty standard one.
It defaults to weighing oils and butters in ounces, but you can change it to grams if those are your normal measurement units. (Addition from Lizzy: These are all weight measurements, not fluid volume)
Find a recipe you like (that's the hard part!) I feel that a 2 pound (approx 910 grams) batch is the smallest you should do when you are starting - weighing inaccuracies in smaller batches might mean you will get lye-heavy soap. By the way, when someone says they are doing a 2 pound recipe, that means 2 pounds of oils, not including all the water, fragrance, and other additives. If you find a recipe that is a lot larger (like the ones at the miller site) do not worry, you can resize it later.
Enter the recipe into the appropriate columns. It has most everything you would want to saponify. You can either add your weights in ounces (or grams), or you can enter the percents if that is what the recipe is in (you can resize it later)
Click the 'calculate lye' button.
OK, now you have a table on the right, with different percentages listed, and lye amounts.
"% excess fat" - this is also called the "lye discount". If you used the amount of lye that is on the 0% row, that would mean you are using enough lye to fully saponify ALL the fats in your recipe. This can be dangerous, because you run a real risk of having lye heavy soap. Scales are just not accurate enough to make sure your amounts are correct.
Many soapers use the 5% discount row. This means that 5% of the fats remain in your soap, without being turned into soap by the lye. (Remember, the chemistry is fat+lye=soap+glycerin)
This means you will have a safe, moisturizing bar. Some people go higher - 7-8%, to get extra moisturizing. The higher the discount, the less lye is used, the more oil is left in the bar.
Some people use a lower discount if they are adding ingredients after trace, like heavy cream. They might go for a 4% discount, so that would leave 4% of the main oils intact, plus 4% of the additional fat from the cream. (It would probably work out to total 5-7% if you actually took the trouble to calculate the percent fat in heavy cream, or milks, and add them into the lye calculation.)
But 5% is a good place to start.
On the left size they give you a range for the amount of water to use when you dissolve the lye. This is based on the total weight of oils, so will be the same for same-sized batches no matter what the contents. When you are starting out, use the larger number or a little bit less. Your soap will take longer to harden up and cure, but it will give you plenty of time to work.
MMS is usually high on the water, many soapers use less water than the largest amount given. This is called the "water discount". Now that MMS gives a range, a good value to use would be right in the midpoint. However, if you are using an unknown fragrance that might not be a behaving one (spices, florals), you might want to use the full amount of water to make the batch easier to work with.
You do not really want to start out with 7 pound batches - that's a lot of oils and $ to experiment with! Try for a 2 or 3 pound recipe - you usually get around 8 bars out of 2 pounds. At the bottom of the page with the lye chart, there are some resize options. If you entered your amounts in percents, this is currently a 100 ounce batch, a little over 6 pounds. Resize it to 32 ounces (2 pounds) or 48 ounces (3 pounds). Your lye, oil, and water amounts will change.
Variations (these can get complicated):
You can use just about any liquid in place of the water. Tea, goats milk, carrot juice.... All of those don't come into the lye calculations.
You can also add things at trace that don't affect the lye amount. For example, I use 1 oz of heavy cream. I don't put that into a calculator. It just means I'm using more oils than the lye can convert.
Some people even add oils after trace, like adding extra castor oil, without putting it into a calculator. That just means that you will have more oils in your final bar that the lye didn't saponify. This would have the same outcome as adding those oils into the calculator and taking a higher discount (using less oils). That sounded more complicated than it actually is. Just add all your oils up front, it's easier that way. If you are doing hot process, this technique works better.