A little background information on how SolidWorks works: you make a preliminary sketch, just like sketching on a piece of paper. Draw some lines, add a curve here or there if needed. No perfect to-scale lines are required, relative size is not even necessary. A very rough approximation, just the general shape, of a part is all you need to begin.
Dimensions and relative size are all added later, and then the computer does all the resizing work for you, changing your rough sketch into a finished, manufacture-ready drawing.
Parts symmetrical to a center line are perhaps the easiest to draw. One side of a symmetrical cross-section is all that is needed to create a fully developed model with one button push.
For example, today I played around with some of the first chapter's problems. The book provided three two-dimensional drawing views of each simple one-piece part: an isometric view, a cross-section, and a right view. The assignment was to sketch the cross section that would be required to revolve the part (none of these required dimensions).
From going through the book work previously, I knew it would only require sketching half of the cross-section. The first one was very simple.
The next one required a little more thought, and I made an error at first. The part is supposed to have stepped holes.
I would have had to add multiple extruded cuts to finish the part correctly, and the first rule in 3D CAD is to create your model using the fewest steps possible. I started over and changed from my original staircase approach (green sketch lines pictured above) to double-sided "steps" (blue sketch lines pictured below) to solve the problem.
The next piece looked challenging at first glance, but with just a few minutes thought and realizing it would be simpler to create the inner surface above the center line and use the center line as the reference for the revolve (just as I had done in the previous 'double-sided staircase' part), I was able to create it quickly with no do-overs.
The next piece gave me a bit of trouble keeping track of which corners were the correct placement for the chamfer and fillets. This was close (below), but in order to correct it, I had to re-do the right end of the part, juggle filter selections, surface vs. edge choices for the chamfer, and delete and redraw the corners more than once.
The final correct part:
It's quite satisfying to realize I have the ability look at a piece, and glean the information I need from it, draw the sketch, and click that "revolve" button to make the part appear. It's especially satisfactory when I make an error, and then have to puzzle it out. Seeing the part appear magically on the screen when I've fixed an error, well, that's right good stuff.