complete list of ionic compoundsCan someone help me with writing chemical formulas for ionic compounds with polyatomic ions?

please give me an idiot proof, step by step general method for finding the chemical formulas of ionic compounds that contain polyatomic ions. I am allowed to use my typed up list of common polyatomic ions for my upcoming test so memorization is not a problem. I will give you some examples that I found confusing

copper(II) dihydrogen phosphate

tetraphosphorus decaoxide

potassium hydrogen phtalate

I understand the basics but I need a complete explanation for my test tomorrow

THANK YOU SO MUCH!
I think the easiest way to find the formulas is this:

1. Find the formulas of both, the anion and the cation

2. Find the valences of the iocomplete list of ionic compoundsns. (e.g. Na+ has 1, Ca2+ has 2, ...)

3. If x is the valence of the cation and y the valence of the anion, find simplest fraction x/y. (e.g. 2/3 stays 2/3, 4/2 becomes 2/1, 4/6 becomes 2/3) Then use this ratio for the stoichiometric ratio of the ions, i.e. combine y(!) cations(!) with x(!) anions(!). (This way you get a neutral compound, without unbalanced charges)

Things will be clear with the examples:

1. copper(II) dihydrogen phosphate
Cation: copper(II) = Cu2? because of the "(II)". The (II) states the oxidation number of the copper ion explicitly. (copper(I) would be Cu?)
Anion: dihydrogen phosphate = H?PO??

Valences: 2/1, therefore use two anions per cation.

Compcomplete list of ionic compoundsound: Cu(H?PO?)?

2. tetraphosphorus decaoxide
(for traditional reasons sometimes called phosphorus pentoxide)

In this case, the name gives a full discription, you don't have to care about the valences;

tetraphosphorus: P?
decaoxide: O??
-> P?O??

3. Potassium hydrogen phthalate

Cation: K? (potassium is in group I, therefore the valence is 1)
Anion: C6H4(COOH)COO-

Result: C6H4(COOH)COOK

You will need to know the prefixes like tetra and deca.

1 mono- or hen-
2 di
3 tri
4 tetra
5 penta
6 hexa
7 hepta
8 octa
9 nona
10 deca
11 undeca
12 dodeca
...