Depending on your art style, one of the defining features of a cartoon panel is the frame. If you’ve read my comic, This is Pi Day, you will no doubt have noticed that I have a fairly strict frame style that contains my strips, no matter if they are one panel, three panel, four panel, six panel, or more.
That style is consistent largely because I’ve set myself some rules and built a small library of pre-drawn frame objects that I re-use. But the style of them is simple enough that I can (and sometimes do) create new or adapt old ones.
The style is also simple enough that following the basic construction of frame object in Inkscape, it could be adapted for many other artists.
So… imagine that you’ve drawn the art for a single-panel comic that you wanted to enclose in a frame.

In this style, your first step will be to draw two overlapping rectangles.
In my screenshot examples, I was not particular about making these uniform but at this stage you could very easily adapt:
- shape and dimensions (rectangular, square, irregular, etc)
- size and bleed (how much of your art is inside/overlapping with the frame)
- color (I choose white)
- stroke width (I use 6 pixels, but different styles could use thicker or thinner lines)
- frame width (the offset difference in size/dimension between the two shapes you draw)

The key step in creating a frame object once the shape, colour, stroke, etc have been established (even though these things are infinitely changeable afterwards) is using the Path > Exclusion tool.
Selecting both rectangles (shapes) and applying the exclusion, you will effectively merge the two shapes into one, with the larger object becoming the outside edge, and the smaller object becoming an inside edge.

The two shapes are now a single shape, and can be manipulated as one.
More importantly, you’ve effectively created a box (you could even call it a kind of outlining “mask”) in the shape of your comic panel with a transparent centre that encloses your art.
I tend to make sure that this is a simple path object as well, by clicking the paths tool and then (with the new frame selected) using the Convert selected object to path button.

As a path, I can apply my own styles now. I usually add some curve to the inside edge using the path manipulation tools.
At this stage I am free to resize, export, or duplicate the panel.

Advanced Steps (other tips and tricks I use)
Try selecting the frame object and exporting it using the “Selection” option (pictured). You should result in an exported PNG file with just your comic panel. Make sure you use an appropriate image size (1200 pixels for web, 3000 pixels or larger for print).
Try creating a wider rectangle and placing three smaller squares (side-by-side with a bit of space between them) inside of the larger frame and using the Path > Exclusion tool. This should create a three panel strip object that you can use to build a three panel comic.
Try drawing a rectangle with a border that falls between the inside and outside edge of your frame object. Select it and send it to the bottom of your objects (Lower selection to bottom button). Keeping it selected, export that. You should result in an exported PNG file with just your comic panel but with the inside frame line only.