Everything from the 1 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Organic synthesis means getting from a starting material to a target using reactions you know. First identify every functional group in each molecule (arene, phenol, alcohol, halogenoalkane, alkene, aldehyde or ketone, carboxylic acid, ester, acyl chloride, amine, amide, nitrile) and predict how each reacts with the reagent given. Then work backwards from the target: what group must the previous compound have had? Each step needs a reagent, conditions and a reaction type (electrophilic substitution, nucleophilic addition, reduction, oxidation, hydrolysis, condensation). Nucleophilic addition to a planar C=O, such as HCN with ethanal, can create a chiral centre as a racemic mixture, but drugs are often needed as a single enantiomer. Yields multiply: the overall yield is the product of the step yields, so short routes are better.
Do not add or average step yields: multiply them. Give conditions as well as reagents (reflux, temperature, catalyst) and say dilute or concentrated. Diazotisation must be kept below 10 °C or the diazonium salt decomposes. Sn/HCl reduction needs NaOH(aq) afterwards to free the amine from its salt. Check whether a carbonyl product is chiral before claiming a racemic mixture, and remember that one reagent attacks every group that responds to it.