The conversion rule
a b/c → (a × c + b) / c
Multiply the whole-number part by the denominator, add the existing numerator, and put the result over the same denominator. Negatives apply to the whole expression — the sign attaches to the numerator of the improper fraction.
FAQ
What is an improper fraction?
A fraction where the numerator is greater than or equal to the denominator (so the value is ≥ 1). It's the natural form for arithmetic; mixed numbers are mainly for reading.
What if my whole-number part is 0?
Then the mixed number is just a regular (proper) fraction; converting leaves it unchanged.