Concept of Part Design Workbench
The idea behind the “Part Design Workbench” is to create a single contiguous part. More specifically a single object. The object may have many “features” but it will generally have one Body. We can create features by adding and subtracting various primitives or by sketching on surfaces and manipulating those sketches into three-dimensional features.
The user interface in FreeCAD can be quite overwhelming. Let’s start out by creating a box in FreeCAD. There are two ways you can create a Box in FreeCADs Part Design Workbench. we will review both.
Step-by-Step Your First Drawing
1. ) Select the Part Design Workbench from the Workbench Selector.
2.) On the left side of your work area there is a Dialog box 
called “Combo View”, within the Combo view there is a Tab called “Tasks”, click that Tab
3.) Within-In the Task Tab you will see a collapsable Dialog listing all the available Tasks. At this point, there should be only one listed. It is “Create body”. Click this item once.
4.) The Tasks tab will change and within “Start Body”, will now be listed “Create sketch”, click “Create sketch”
5.) Tasks tab should now show the “Select feature” dialog. Click the XY_Plane (Base plane). This will “Attach” your design to the XY Plane of the drawing. Then click “OK”
6.) Your view will be switched to the “Sketcher Workbench” You will now be able to add Sketcher Geometries.
7.) From the geometries, toolbar Click the “Create rectangle” icon. It is a white square with red dots on two corners.
8.) Click once near the center of the drawing and click a second time in the upper right quadrant. The size of the rectangle does not matter at this point.
9.) You should now have a white rectangle with four red dots at the corner. From the “Tasks” tab click Close.
10.) The tasks tab will now have a list called “Sketch tools”, click on the Pad item.
11.) Your two-dimensional “Sketch” will pad into three dimensions. You can adjust the “Length” parameter to change the length of the Pad in the Z direction.