The key aspect to remember when you are adding or subtracting fractions is to understand that you need to have the same size pieces before you can do any computations. Finding a common denominator is creating equal size pieces. Without breaking your pieces (denominators) into equal sizes you are unable to compute. Notice that finding a common denominator is the same as finding common multiples. See the videos from www.khanacadeny.org
When adding mixed numbers you do not have to convert to improper fractions. (I notice many students doing this and it will work but it complicates your problem and leaves you open to more errors.) Remember the commutative property allows you to reorganize your addends in any order. You will see this in the video below. Therefore you can add your whole numbers and your fractions separately. Keep in mind you still need to have the same size pieces.
Part of your assessment will be to determine what operation to use when solving word problems. Remember we developed many examples of word phrases that give you clues. But it is not wise to rely on memorizing them to solve all word problems. Some word problems will not fall into any of the categories we discussed; therefore, you need to make sense of the problem. One strategy is to substitute the mixed numbers with easy whole numbers. This will help you determine whether or not your approach is making sense.