Hi Danny. Welcome to TFP!
Sorry you inherited a green mess. :-( There are a lot of people on here that have been in your shoes. I had a green mess that was my own doing. I had no idea what I was doing. I was trying really hard. I had a wal-mart test kit and I really thought I was understanding it. Then I found TFP and saw what I had been missing.
There is a ton of information here and a lot to read so it's pretty normal to be overwhelmed. But that's why were here in the threads to answer all the questions that will come up when a new member starts trying to absorb all of this information at once.
I think your confusion on the SLAM process is probably due to the fact that the pool store has given you a formula (which has to do with FC and CC) and that is just not a workable formula as you noticed.
The biggest thing to understand is the CYA/Chlorine relationship. I know it's in the links that have been posted but a very quick simplification is that CYA (Also known as stabilizer or conditioner) is in a lot of "shock" products that are sold for pools as well as the chlorine pucks that they always recommend to float in the pool (Or in an inline chlorinator)
CYA is good. At the appropriate levels. It buffers the harshness of chlorine and protects it from being burned off by the sun immediately. The problem is...it builds up. It pretty much doesn't go away. Chlorine is consumable and gets used up while it "works" which means it needs to be replenished. So what happens is your CYA can get so high that the chlorine is pretty much too protected and can't work. Voila, green pool!
The way to prevent that is to add chlorine that doesn't also have CYA in it. That is why we almost exclusively use liquid chlorine (aka bleach) as it has no CYA in it and only adds chlorine to the water. (and a tiny bit of salt which has no negative effects)
We have a chart that shows what level your chlorine (FC) has to be kept at to be effective in relation to your CYA level.
It is very very important to know that CYA level. It is unfortunately the test that pool stores are the most inaccurate with.
Everything for a SLAM is based on that level. How can a person get their chlorine to an effective level if they don't know what level that is? Without a reliable test result it's only guesswork.
Once you know the CYA level, all you have to do is look on the chart to figure out what level your chlorine needs to be kept at to rid yourself of the green mess.
To keep that chlorine level you test and add liquid chlorine, aka bleach, as needed until the pool is clear. To test that level you need a chlorine test called an FAS/DPD test which can measure your chlorine levels all the way up to 50. Most kits only go to 5 (which is not high enough) and don't have the accuracy required.
This is why a proper test kit is needed. This is also why we have specific test kits we recommend. The pool store will sell you one but it will almost never have the right tests in it.
We're always here to help! Read around and you'll find a bunch of active threads where people are going step by step to clear their pools. (Some of them have even started with tadpoles and frogs!)
Post any questions you have and someone will jump in to help. If you decide to move forward in the cleanup get us some accurate test results (from a kit with the recommended tests) and we'll help you step by step! You'll learn along the way and you won't believe how easy it is.
Welcome again!
p.s. random useless fact, I have a horse named DannyBoy. Good name!