In school, they really don't spend enough time teaching flat sketching.
A "flat" is the technical rendering of a garment that is shown to the pattern maker and featured in the "technical package" or "tech pack". A good flat indicates proportion, construction, stitching details and overall aesthetic of a garment. From a good flat a pattern maker or "technical designer" should be able to accurately advise all "specs" in order to make an accurate first sample. As an assistant designer starting in the industry, a strong ability in flat sketching is essential as this is a key responsibility in that role.
Start with proportion. Usually, a flat is done in a size 8 or size 10 proportion. The flat should looks as if you took the garment and spread it flat on the table. Flats can be drawn by hand or on the computer... often using adobe illustrator.