A level Chemistry OCR HELP

I need help with these types of question :
How many structural isomers of C6H14O are tertiary isomers?
i know the answer now obviously but how do i figure these types of questions where they ask how many isomers there are.
please help

This is how I would probably work through it, if this helps?
Firstly, I'd separate the -OH to make checking the H atoms easier (C6H13OH).
Then I'd start with a 6C chain, then 5C, 4C etc. until there are no more possibilities (ie. they start repeating).
We know that a 6-carbon chain won't work as that would only result in primary or secondary alcohols.

I had a look and briefly explained what I did in the attached two images. Hopefully the notes make sense!
Btw, I added the blue H atoms just to help me check I had 13 (without the -OH), and remember to consider cyclic structures (which aren't possible here as that would require one less H atom).

I've just remembered that you could add ethyl groups (-CH2CH3), I think this would be less likely, but if an ethyl group replaces any of the methyl groups (must be the methyl attached to the -COH) and one carbon taken away from the straight chain (of any of the three isomers I drew), then we can see that the ethyl group becomes part of the longest chain (so it's no longer an ethyl group and the compound ends up being one of the already drawn isomers).