You can hold the Shift key to constrain movement when laying down points with the Pen tool. However, with the graphic you're describing, I think that starting out with a guide (and having 'Snap to Guides' turned on) is a great idea.
For the black lines, I'd draw simple rectangles or lines with varied strokes, then make a copy of the yellow oval (the bee's lower body), place it above those vectors, and apply it as a mask (using Modify > Mask > Group As Mask). This would make it easier to create the rounded edges and match them to the existing shape.
But that's unrelated to your original question. My general advice is, if a tutorial leaves you with too many questions, move on to another one that's more educational.
(Incidentally, instead of masking, you might use one of the Combine Paths commands to create similar visual results—using the oval shape as a cookie-cutter for the black lines.)