For start we go to part design! in Catia software (my version: V5R20) select Start> Mechanical Design> Part Design. We’ll see something like below image, here we have three Planes like Cartesian coordinates (xy, yz and zx), they are perpendicular to each other (for more see Wiki : Cartesian coordinate system) and we could use any of them to draw in 2D as we click Sketch button in right side (looks like pen on a page).
Below image shows sketch environment, in right hand column we could use tools like: line, circle, point and etc., by just a click on them. Every shape has different way to be constrained, constraint tool could reduce degree of freedom “DOF” by defining appropriate dimension (e.g. A circle will constrained by defining x and y of center point and it’s radius or diameter). To control a sketch we need to reduce DOF, otherwise that will be a nightmare in complex sketches. If we add unnecessary constraints, color would change into purple then we have to remove one of them.
After sketch we click on “exit workbench” (in right side column looks like top arrow in plane). In part design environment we have tools called “Sketch-Based Features” 1st one is Pad if we click on it and choose a sketch that we have already drew, we could extend 3rd dimension perpendicularly straight. If we check “Mirrored extent”and Length is 2 cm we will see 4 cm length actually.