Hi,
Creating a Drawing from a Sketch completely defeates the purpose of Modeling.
Your workflow should be, create a simple sketch, use the tools to model the part, then use the part to create
the drawing. Since you are using the actual part in the drawing, you can then use the drawing tools to get your
various dimensions and cross sections. They will be precise and accurate. A fully defined sketch "should" also
be precise and accurate but it is only a picture, a modelled part can be created but pictures sometimes cannot.
The link that @Beyondforce has provided is the official Fusion documentation and is an excellent place to get
up to speed with how to create drawings and other stuff.
The Fusion drawing package is powerful and useful and it is well worth learning, it makes creating drawings very
quick, easy and precise. The other Fusion tools also use the model and Gcode can be directly generated for CNC
or 3D printing. I don't think it is possible to do this with a Sketch.
What you want to do is use the model you have, import that into the drawing, then do a section of the model using
the drawing tools. This process is explained in the documentation link you have been provided with.
Cheers
Andrew