Mr Hegarty takes you through how to add and subtract fractions using bar models. This is aimed at KS3 students, but there is plenty that we can take out of his explanation, especially at the beginning!
Adding fractions with the same denominator - Corbettmaths
Adding fractions with the same denominator with Mr Corbett
Fractions Addition Subtraction - Corbettmaths
Adding and subtracting fractions with a different denominator with Mr Corbett