Sheeetcam is not a design program. It is only intended to make your design into a toolpath and post process it into a .tap file that Firecontrol can use.
Spend some time learning the basics of sketching in CAD. 2D is much easier than 3D design, because you only have to worry about a flat design. The most important thing to learn in creating a CAD sketch is that everything has to be “constrained”. You have to set a distance(vertical and horizontal) from a fixed point like the origin or another fixed point in your sketch. Circles and arcs have to have the radius set and the center point constrained to a location. Youtube has plenty of videos for learning these things. I learned to use Freecad by watching Youtube videos and following along until I understood the principles. All parametric modeling programs have different names for the tools, but the basics of sketching and constraints are the same.
Inkscape is a graphic design program and the drawing style is much different than parametric modeling. It is more useful for art designs, where CAD is better for accurately dimensioned parts. Logos by Nick on Youtube has a ton of videos on how to draw in Inskscape.