How about trying to make $90$ using the digits $1-8$ and leaving the $9$ for adding on at the end?
A good place to start may be to push the $1$ and $2$ together to make $12$. What will you still need to add onto $12$ to make $90$?
Can you make $90$ simply by adding the remaining digits together? What could you do?
Perhaps you could try pushing the $2$ and $3$ together to make $23$? What do you need this time to make $90$?