These are some steps on drawing components of a diaper with eight steps.
Part 1: To help get a starting point, I add a belly button in the middle of the belly. This is optional, but this helps me out.
Part 2: Draw two lines beside the belly button for the tapes. The left line is further apart and the right line is closer.
Part 3: Draw rectangles on the middle of the lines for the tapes. The shape on the right is shorter, and I have the edges stick out of the red dotted line.
Part 4: For the perspective, I placed some curves around the waist and the legs. I placed 2 V-shaped curves besides the ends of the legs and two curves on the waist. One is across the waist and the other is connected to the lines that hold the tapes.
Part 5: For the outside parts, label connection points where you want the straps to connect the tape lines.
Part 6: Draw along the connection points and along the ends of the legs. Then, remove the lines circled in yellow. Draw inside the tape of the colour of your choice.
Part 7: Fill in the diaper's area with colour and for thickness effect on the tapes, make the shapes squiggly to the front.
Part 8: For thickness effect, draw lines around several parts of the area. Then add a squiggly line above the waist to draw the tip of the waist shield.
Optional tips: You can add designs and a wetness indicator to make it stand out more. There are two types of wetness indicators. One with a yellow lines that turns blue when wet and the other has designs that fade when wet.

Update: If some parts are too small to see, you can view a larger version here: https://www.pixiv.net/en/artworks/96623395