Switch to the rooms layer to create the first part of the level. Don't use the rectangle tool to create a room or corridor. Instead, use the bezier line tool to create an outline of connected rooms and corridors, connecting at least two of the geomorph connection points (entrances/exits on the geomorph sides.) Use a white fill color with no stroke color, but for comparison, I've included a version with a yellow stroke color.
|corridors + room|
You may have to switch to the node tool and "tug" each node a bit so that it snaps to the grid properly and lines up with the entrances/exits.
Geomorphs are improved if you add a bit of three-dimensionality with corridors or rooms above the base level. For these additions, set the stroke color to black and make the stroke thickness 1. You can set stroke width without opening the Fill and Stroke tool by right-clicking the number next to the stroke color in the bottom bar. (It took me at least a year or two of using Inkscape before I figured this out.) To get the walls of this upper level to line up, you can switch to the node editor, select each line segment, and tap an appropriate arrow key once or twice.
Add dashed lines to show walls that pass under the second level. The first dashed line in the Fill and Stroke tool will work fine.
To cover up the solid line leading to the left end of the south side, use a borderless white rectangle.
Once you are satisfied with the layout of the rooms and corridors for this geomorph, lock the layer and switch to the layer above (details). This is where you will add things like staircases, doors, traps, pits, and other symbols.
|parallel line segments|
An easy way to make a staircase symbol: Off to one side of the document, make a line segment, copy it, and paste it several times. Align them all, then select all the lines except the top line. Shorten the lines by a tiny bit (easiest way: use the W field in the toolbar at the top,) then deselect one line and repeat, so that the lines get shorter and shorter as you go down. Once you've shortened all of them, select all the lines, center them horizontally with the Align and Distribute tool, make the vertical gaps equal, and adjust the stroke width to 0.75. Group them, then adjust the size of the staircase symbol. Reposition the staircase where needed. Copy the symbol if you need more than one staircase.
|group and resize|
A good way to indicate a ladder or trapdoor: Make a circle (no fill, stroke width 0.75) and a line segment equal to the circle's diameter. Center the line horizontally and vertically in the circle, then use the bucket tool in one half of the circle to create a shaded half. Group the result to make the symbol and position it where needed.
Simple door symbols: Make line segments (stroke width 1) to seal off any room entrance or corridor where you want a door. Draw a rectangle (rectangle tool or bezier line tool) with a white fill. Adjust one of the dimensions using the W(idth) or H(eight) field in the toolbar so that it is one-half or one-third the other dimension, then change the stroke width to 0.75 or 0.5, whichever you prefer. Center this horizontally and vertically over the wall segment using the Align and Distribute tool.
There's certainly more that can be done: more symbols, or adding text on the labels layer (not generally used with geomorphs, but you might use this same template for one-page dungeons or pamphlet dungeons.) You can draw little pictures, either directly in Inkscape using the freehand tool or with real pen and paper, then snapping a pic, importing it into Inkscape, and correcting colors with a threshold or levels filter/extension. There's also ways to add fancier background fill patterns.
If anyone would like to see tutorials for any of those techniques (or how I made something in one of my geomophs or other maps,) let me know.