HOw would I write nickel ll chloride and chromium lll nitrate?

Simple...

Write the symbol of nickel and chloride (it has the same symbol as chlorine). Put them side by side, write down their valency under their symbol (you're given the valency of nickel as 2. Then 'cross multiply'

{Symbol of nickel} {symbol of Chloride}
OOOO\searrowOOOOOOO\swarrow
OOOO2OOOOOOOOOOOxOOOOOO
2{Symbol of chloride} x{symbol of Nickel}

Then, put the Nickel, with the subscript of x (which you replace by the valency of chloride) and then the symbol of chloride with the valency of nickel as subscript, that is 2. That metal non metal order is only conentional.

Try the other one now.