Double the number, then reduce by one-tenth the doubled number, then add 32
Example: To convert 25 C double it to 50, reduce by 5 to get 45, then add 32 to get 77 F.
The reason this works is because 9/5 = 10/5 - 1/5. And the nice thing about this is that it is an exact conversion not an estimation.
It’s even easier to skip the “reduce by 1/10” step and you still get close. It gets less and less accurate the larger the number becomes, but most people are doing this for day-to-day outdoor temps and it’s more than good enough for that to get close. Maybe don’t rely on it for baking, though.
edited to add, also instead of “adding 32” it’s often easier to think about just adding 30.