Starting with a blank screen in a design can be challenging. Even if a rectangle is drawn as the first step in a sketch, there is no sense of scale. Adding dimensions to a sketch helps show scale.
Another way to get started with a design is to use sketches that have already been drawn as guides. This is especially helpful if the sketches are in from a third angle projection or an isometric view. Using sketches as a guid is also helpful when designing organing forms or surfaces. Perhaps you made a foam prototype. This can be photographed and then brought into Fusion 360 as a guide. Photographs of different views of a cardboard prototype can also serve as an inserted canvas.
Fusion 360 allows the insertion of “canvases” on planes that can be scaled to the correct size and then used as a guide to make a model. This helps with laying out the proportions of shapes. It is not necessary to insert sketches but it is a helpful option. The videos below show how to insert and scale a sketch as well as how to insert an svg or dxf that might have been drawn in a vector program.