Imagine the cat in your profile picture opening its mouth wide like a C, ready to eat two sardines: C SS

"Necessary" is a famous "spelling demon," a word whose spelling is hard to remember, even for native speakers. My advice is to invent your own memory trick. It doesn't need to be very clever. It has to be based on the particular mistakes you make. Now, I would never write "neseccary" or "nessecary" because when I look at those patterns of letters, I feel that the "cc" or "c" would be pronounced like a "k." And I know the word isn't pronounced "ness-uh-kerry." And for some reason it feels natural to end in -ary rather than any other alternative. So the only real question is whether it has one "c" or two. The kind of bizarre, unnatural trick that might work would be "it isn't spelled the way I think it should be." The reason this could for for me is that to me it looks much prettier and balanced to have two c's matching the two s'es. So if I can remember that "the right spelling is the ugly-looking one" or "it isn't spelled the way I think it should be," I can translate that into "one c, two s'es." Another stupid trick that might work for me is that I don't have any trouble remembering the spelling of the word "cesspool," so I might be able to say "'necessary' is spelled like 'cesspool,'" or, more cleverly, "a cesspool is necessary," and use that as a cue that "necessary" contains "cess."

There's an English saying: "It's your neck on the line," or more often just: "It's your neck." Very literally, this means that, if things go wrong, you will be hanged; but, in practical use, it only means that if things go wrong, you will be held responsible. I remember the the first 's' sound in "necessary" is a 'c' by thinking "It's your nec(k) in 'nec-essary' - if you don't use the necessary 'c' in the first syllable, you will be responsible for misspelling the word." Then I don't have much trouble remembering that the other 's' sound is the other letter that sounds like 's' - that is, 's' - and that I need to use two of them. This is just my way to remember the spelling; I don't know if it will help any one else. :)

Another approach is to be familiar with the difference between "c" and "cc", since your examples show you're having some confusion there. I wanted some source to back up my intuition, so I checked Wikipedia here under soft C. Specifically you can search the page for the symbol "⟨cc⟩": https://en.wikipedia.org/wiki/Hard_and_soft_C A double "c" is usually going to be pronounced as "ks" or "k", except in a few words. Those exceptions don't matter right now. The point is this: if it were spelled "neccessary" or "neseccary" then you'd probably pronounce the "cc" like "ks" or "k" instead of like "s". Sure, that doesn't help in memorizing the rest of the word, but it gets you half of the way. Not to mention it hopefully helps you remember how to spell some other words too.

I always say 1 coffee, 2 sugars, 1c and 2 s in the spelling.