Mnemonics are a way of remembering lists by using the first letter of each word in the list as the beginning of some other word, and those words making up a sentence or some such. (You can google [mnemonic anatomy] and find a bunch of really cool ones.) However, I have found that if I just focused on the lists of words (like, the cranial nerves in order), I actually learned them, without having to learn a mnemonic first.
The "grid" everybody talks about is a chart you make of the stuff you have to do at the beginning of each PCS--each patient you will care for in your alotted 2-1/2 hours, organized by body area, body system, or "area of care," or however it works best for you.
I didn't use either one of them, although I have tried in the past. It just wasn't for me. Here's how I learned them. It must've worked, because I still know them.
Having experienced the CPNE first hand, I can tell you that you will probably be more confident if you just "learn" the critical elements, and then, at the CPNE, highlight on your report form which you will give back (yes, you can do that) the parts you need to do. I found this to be much, much less work, and no chance of forgetting something I forgot to put on my grid or list.
How to learn the critical elements? Look at each of the task areas (i.e., respiratory assessment, mobility, fluid management, etc.) one by one--don't try to do this for the whole shebang in one attack. For each of those, there is a detailed list of "what to do." You'll probably notice that you know some of those "elements." So don't worry about the ones you already know. Look at those parts you don't know. Gently read over them a couple of times.
Are they starting to get a little familiar? You can anticipate them as you read, can't you. Keep doing this a couple of more times.
Then get some 5x8 note cards, and write the critical elements of each of the "areas of care" down, one at a time. Remember, there's like 24 of these things, you only want to focus on one of them.
I found that by the time I had written my third set of cards, I knew the stuff and no longer had to look it up when I was writing the steps to do them.
Suddenly, they were all done and I didn't have to look at the study guide for any of them.
And that's how, my dear.