multiply denominator by denominator and vice versa
Yes you do.
you do not do that
None. Multiply numerator by the numerator and denominator by denominator.
Just multiply the two denominators of your fractions, the answer you get is a common denominator.
You Ned to find a larger common denominator or multiply the denominators to gain a common denominator.
When multiplying fractions you multiply the numerator to the numerator and the denominator to the denominator. For example: 2/3 x 4/5 = 2x4/3x5
When you multiply fractions just line them up (denominator and denominator, numerator and numerator) then multiply across. Simplify.