Parcels pile up at this time of year, and most of them get opened the same way: a key along the tape, a tug, and a hope for the best. It usually works. Now and then it doesn't, and the thing you ordered ends up with a nick in it.
Find the seam first
Look at the top of the box and find where the tape runs. Most boxes have one long seam down the middle and two short ones at the ends. Cutting along the tape, rather than across the flaps, keeps the blade away from whatever is packed just under the lid.
Keep the cut shallow
Cardboard is thinner than it looks. You only need the blade to go through the top layer. Press lightly, let the edge do the work, and stop at the end of the seam instead of pushing on. If the box is packed tight, lift one flap with your other hand as you go so you can see what's underneath.
Work at a table, not on your lap
A flat surface at about waist height is steadier and keeps the blade pointing away from your legs. Clear a space first. It takes ten seconds, and it is the easiest way to avoid a slip.
Keep the packaging for a few days
Check the contents before you flatten anything. If something isn't right, you will want the original box and the label for sending it back. Once you're happy, flatten the box so it takes up almost no room in the recycling.
Use a blade that suits the job
A dull knife needs more force, and force is where accidents happen. If you open a lot of parcels, a sharp, shallow-cutting tool with a guard is worth keeping near the front door. Replace blades when they start to snag, not when they stop cutting.
If you open a lot of parcels, our Cordless Rotary Scissors are made for this job: a round blade, a flat foot that rests on the box, and a charge over USB.