I do this a lot.
Import the scan into Gimp.
Using the selection tool, select the pattern you want.
Copy and paste it into Inkscape.
Use trace bitmap to create an SVG.
Delete the bitmap the you pasted.
Select the SVG, resize it, and move it to where you like.
You may need to do some editing in Gimp, depending on the quality of the scan. Colored backgrounds, shading, and print showing through from the back of the paper can cause problems that need to be fixed in Gimp. The scan is a bitmap, and Gimp is a bitmap editor. Patterns printed on magazine pages are the worst as they are printed on cheap, thin paper.
If the scan is clean and clear, this is simple. If it isn't, you will have a great learning experience as you try to figure it out.