Fit a paper cover to a notebook
Fit a paper cover around the closed notebook so its spine and tucked flaps have enough room.

Measure with the notebook itself
Place the closed notebook on a sheet and check the height first. The cover should finish flush with the top and bottom or project only slightly. Large overhangs quickly bend during use. For width, allow for the front, spine, back and two folded flaps. A thick notebook's spine is a real measurement, not an amount to ignore.
Wrap the sheet loosely around the closed notebook. Mark the outer edges of both covers and add a few inches for the flaps before cutting. Estimating from an open notebook can produce a wrapper that becomes tight as soon as the book closes.
Tuck the covers in without glue
Fold one side flap inward along its mark. Slip the front cover into it and wrap the paper around the closed spine. Locate the second flap directly on the notebook, allowing enough room that it does not pull the back cover outward. Remove the wrapper to firm its folds rather than pressing a sharp crease into the notebook's pages.
Slide both covers into their flaps and open the notebook slowly. If it resists opening comfortably, give the wrapper more room. Tape on the original cover is not a good cure for a tight fit. Thin notebooks may work better with narrower flaps because fewer layers overlap inside them.
Plan for labeling and wear
Write the title while the wrapper lies flat. A small separate label can be changed later without replacing the whole cover. On patterned paper, check that the front and back motifs face the directions you intended after folding. Some designs inevitably turn on a flap.
A loose paper cover protects against light scuffing, not water. If it gets damp, remove it and let the wrapper and notebook dry separately. Replace badly torn flaps rather than reinforcing them with many thick pieces of tape. Keep a worn cover as a measuring template if its fit was successful. Its creases and wear marks also reveal where the next version might benefit from a little more clearance.