There are two types of drawings used in the fashion industry, both used for very different purposes; the Design and the Fashion Flat. As a home-sewer patternmaker, you don't need to be able to the first, but you do need to draw what you are wanting to create to some extent, in a technical illustration.
The Design: A Fashion Sketch
In the Fashion Industry, a Designer (not the patternmaker) comes up with a design. The fashion sketch is drawn on a stylised figure called a croquis; while you may be able to see the design lines on the croquis, the focus at this stage is the style and hang of the garment, the pattern on the fabric, etc. Usually these are hand-drawn graphics that are very artistic. The croquis generally look nothing like the real human figure, for example the legs may be extremely long and the waist non-existant. This design is the concept. It will be in colour and include the fabric design to be used.
Figure 1 shows an example of croquis that I use. I create my croquis from photographic images so they look human-like, and don't look like the very artistic and stylized croquis that are used in the industry.
For examples of fashion croquis, look at the drawings on commerical patterns envelopes, or do a Google search on Fashion Sketches, you will find plenty of examples of the drawings that are used in the fashion industry.
As a home sewer making your own designs, you do not need to be able to draw fashion sketches. You do need to be able to draw an idea of what you are wanting to create, but what you do need to do is draw some kind of technical illustration, which is called a Fashion Flat, and will be covered next.
The Technical Drawing: Pattern Flats
While the fashion croquis sketch is the concept for the garment design, it isn't what the patternmaker refers to when making the pattern. The patternmaker needs a technical illustration, which is called a Flat, to make a pattern. Along with the flat there would be a Specification Sheet which would give detail that may not be obvious from the Flat.
The Flat will be in scale, have a front and back view of the garment, and contain details such as dart lines, dart equivalents, design lines, pockets, buttons, zips etc. There is enough detail in the technical flats for the patternmaker to be able to make a pattern. The specification sheet gives detail that you can't see from the drawing, for example, you cannot tell by a drawing the exact fabric, how wide the hem is, what if any, lining, facing and interfacing is required, the number of buttons, the length of the zip. etc. The patternmaking does not make these kinds of decisions, the patternmaker implements the design that iis given. The patternmaker needs to be able to read the flat image, together with the specification sheet, and create a pattern from that information. The Flat is not usually colored, or have the fabric pattern included.
A home sewer making their own patterns would need to determine all this information for themselves, and while it is not essential to create a Specification Sheet, you need to have a clear idea of exactly what you need for your garment. At the minimum you should have some kind of technical drawing that shows the design lines front and back, and notes about the detail that may not be evident from the drawing.
The main difference between a patternmaker in the industry just makes patterns; someone else creates the design and makes decisions about fabric and detail, someone else does the cutting, and someone else does the sewing. This means the home sewer-patternmaker has it more difficult in some ways, but easier in others. You are not relying on others, but you need to make sure you can rely on yourself. Don't assume that having an idea or a concept in your head is good enough. Don't assume you'll remember next week what you have decided to do today. Otherwise you may find yourself coming back to pattern you started making last week and have no idea what you thought you were doing.
Understanding or Reading a Flat
So, the very first step in creating a pattern is to have a flat which shows the design of the garment, and you need to be able to read or understand it. If you are a home sewer learning patternmaking, you will also need to draw it first. You don't need to drawing to be artistic and you don't have to be good at drawing, you just need to indicate all the stylelines and information that is needed to know what pattern pieces you will need to create.
This is continued in Reading a Flat.