Hardly anyone is the "perfect size" and I am no exception. I haven't gone into much detail before, but I regularly make sizing adjustments when sewing. I am tall (5'11") with a long waist, full bust and narrow shoulders.
The first thing you should do when starting to cut out a pattern is inspect the "FINISHED GARMENT MEASUREMENTS" (see above) which are usually printed on the bodice front pattern piece. Depending on the design of the piece and the ease allowed, you may not need to make any adjustments.
The best tip I ever read was that if you have a full bust and narrow shoulders, you can cut the armholes and neckline a smaller size than the sides. For Simplicity 1873 (back piece shown above) I cut a size 14 for the armholes and a size 16 for the sides and shoulder.
I made the same adjustments for the front and sleeves. Sometimes, I also add length to the bodice to adjust for my above average height. Sometimes, I also cut the sides of the back pattern piece at a smaller size. I often end up taking in the back of a shirt or dress more than I need to on the front.
The important thing is to get to know your body and what flatters you (trying on ready to wear is an easy way to do this). For example, I know that a dress or top with an empire waist is very hard to fit my body and will probably make me look rather chubby. But I also know that a dress with a fitted waist at the natural waistline will be flattering. Then, learn the pattern adjustments that will make your garments fit your body.
See where else I've used Simplicity 1873:
Peacock Print Pleated Dress
Cut Out Back Seersucker Dress