This is going to be fairly simple to do.  The first thing we will make is your toilet template so you can make different ones without starting over.  So open a new transparent image, 300x350.  With the way some of these pieces will go together, I found it easier to flood fill the background with a solid color.  You may prefer to work on the transparent background.  To make this template you are going to use different solid colors for the different pieces that make up the toilet.  Make sure both of your Styles swatch boxes have the same solid color in them.  After choosing your first color, white in my case, click on the Preset Shapes and choose the Rounded Rectangle, Retain style unchecked, Antialias, Create as vector, Line width 2.  Draw a selection similar and as big as the one shown below.

Note: When working with a vector shape, if you lose the edit frame, the boxed line with little square blocks around it, click on the Node Edit tool on your Tools Palette.

While the vector node editing box is still active, hold down the Ctrl key on the keyboard and squeeze the bottom inward.  Deselect and convert to a Raster Layer named tank.  Click on the Preset Shapes again using the Rounded Rectangle again, same settings.  Choose a new solid color.  Draw a rectangular shape on top of the tank, extending over it.  Make any adjustments before converting it to a raster layer and naming it tank top.

Now for the bowl top.  Click on your Preset Shapes again and choose the Crescent moon shape, Retain style unchecked, Antialias, Create as vector, Line width 2.  Remember to change your solid color again, top and bottom swatch.  Draw a selection similar in size and relation to your tank.  Rotate to the Left, 90 degrees.  Make sure that All Layers is unchecked.  Now adjust the moon by squeezing in to be more narrow, and stretching it out so the pointy ends extend well beyond the tank as shown below.

Convert to a raster layer and name it bowl top.  Trim the edges to meet the tank.
Position it with space between it and the tank as shown below.

Click on the Preset Shapes again and choose the Ellipse, Retain style unchecked, Antialias, Create as vector, Line width 2.  Change your solid color, both boxes.  Draw a circular shape as shown below.

Move this layer under the bowl top layer.  Adjust it to meet the ends of the bowl top.  Convert to a raster layer named bowl bottom.  Select from the bottom corners of the bowl top and delete while still active on the bowl bottom layer.

It's time to make the toilet base.  I'm going back to white for my solid color.  Click on the Preset Shapes and go back to the Rounded Rectangle, same settings.  Draw a selection similar to the one shown below.  Hold down the Ctrl key on the keyboard and squeeze the top inward.  Convert to a raster layer named toilet base and move to the bottom.  Once again click on the Preset Shapes, still using the Rounded Rectangle.  Hold down the Ctrl key on the keyboard and squeeze the bottom inward.  Convert to a raster layer named toilet back and move to the bottom.