Step 1: Select a solid cylinder and a hollow cylinder shape and place them on the workplane.
Step 2: Place the hollow cylinder inside the solid cylinder.
Step 3: Group them together to create a hole inside the cylinder.
Step 4: Increase the height of the object according to the length of your pen.
Step 5: To create a Cap of cover, select another workplane using workplane icon and place it on the tap of cylinder. Drag a cone on to the workplane.
Step 6: Invert the cone and place another cone and change it to hollow.
Step 7: Invert the hollow cone also and place it inside the solid cone.
Step 8: Group them together to create a hole.
Step 9: Invert the shape and place it on the top of the cylinder such that it gets fixed inside it.
Step 10: Remove the second workplane and your cover is ready.