My grandparents have a sink that is very old. A small plastic piece from the stopper had broken off. Replacements didn't exist and a new sink would have cost way too much money. So I decided to design and print the replacement.

Supplies

CAD Software (I used Fusion 360)
3D Printer
3D Printer Filament (something durable like PETG)
Calipers

Step 1: Measuring

First pull out you calipers and take some measurements. You can write them down on a sheet of paper or you can do it digitally.

Step 2: Designing the Base Structure

I started by basically stacking cylinders on top of each other.

Create a sketch on the bottom / xy plane
Draw a circle around the origin with the measured diameter from Step 1
Extrude the circle the measured amount from Step 1
Create another sketch on the top face of the cylinder just created
Draw a circle around the z axis with the next measured diameter from Step 1
Extrude this sketch the desired amount
Repeat until the base structure is done

Step 3: Cutout to Allow Flexibility of the Part

The injection molded piece had a hole to allow a portion of the part to flex in and out in order to open and close the sink-plug. I designed this cutout next:

Create a sketch on the front plane
Draw a point where the center of the cutout will be
Draw two perpendicular lines and make sure that they cross in the point you created
Draw a rhombus connecting these points
Do a symmetrical cut through the entire part

Step 4: Details

Add fillets near sharp corners for rigidity
(Optional) We will print the part on its side for optimum strength. This results in overhangs which need supports. You can cut off a portion of the cylinders (as seen in the picture) to make sure the part prints support-free