Spell them when they’re less than 10 (unless they are percentages).
The number ‘ten’ can either be spelt or written in numerals.
Spell a number if it’s the first word in your sentence.
It means ‘by which’. In most cases, you probably just mean ‘where’.
Don’t use them because they aren’t allowed. Or perhaps I should set a good example and say: Do not use them because they are not allowed.