Note: Mods, if this should go to egl_comm_sales, let me know.
Items you will need
Good Choices: Shipping boxes (From the PO), Packing boxes (like from Uhaul).
Poor Choices: Shoe Boxes (very few are sturdy enough), Food boxes (Cereal, soda, pasta, etc), Non-water resistant boxes (like with dyes that can run)
-An envelope (if a box is not needed)
Good Choices: Padded envelopes (especially from the post office), Tyvek envelopes.
Poor Choices: Plain paper envelopes, Manila folders.
-Bubblewrap (not always needed)
Can be large or small. Larger items need large bubbles, either size is fine for smaller items.
Good Choices: Shipping tape (like from the PO, usually clear), Parcel Tape (the brown kind)
Poor Choices: Masking tape (adhesive is too weak), Scotch tape (WAY too weak), Duct tape (many POs ban this), Painter's tape (too weak and just silly :P)
-Newspaper/Tissue paper/Office paper
-Bags (Avoid trash bags or others that can easily melt).
1. Set up your box and tape up the bottom along the seam. Make sure your item(s) will fit inside comfortably. Non-delicate items can make the box bulge a little bit, but if there is going to be major gapping when you close it, you need something larger.
If it's a delicate item, make sure there is room for protective packaging.
2. Put in paper so it covers the bottom. I just like to do this so I can wrap the items inside, this isn't 100% necessary. If you use two larger sheets, you can use the excess sticking out to wrap down.
3. Bubble wrap any items that need it. I would bubble-wrap shoes if they aren't being sent in their box or something close to the same size. Remember, bubble face outwards. If you wrap with the bubbles facing in, they aren't going to be cushioning anything.
4. Put the items in something. I like to use clean bags, especially for clothing items. Fold them over and tape and the items inside are protected from moisture. You can use brand bags, new bags or shopping bags, just make sure they are clean and have nothing inside. Some like to wrap the items in tissue paper before putting them in a bag, which is fine.
5. Put the items in the box. If you have extra space, wad up paper to fill in gaps. You can also use styrofoam, packing peanuts, etc.
6. Hold the top close and shake your box. If your items are shifting around, you may want to pack tighter. This isn't very important for clothing that cannot be broken, but for other things, you don't want it to be shaking around.
7. Tape the top shut along the seam. Then tape all the way around the box the opposite way so you have a cross on the top and bottom. This helps hold the package closed, even if some tape is ripped. If you have a little bit of gapping, tape over it over and over so it's a little more closed up.
8. Label accordingly. If you're writing right on the box or using paper labels, you should carefully tape over so that moisture doesn't make it illegible.
When re-using boxes, you may want to scribble over any names on it/remove any stickers/etc. Not all places care (UPS doesn't) but some will make you do this.
9. Now it's time for the ultimate test. Take your package and drop it down a flight of stairs. If you are afraid to do this, you need to re-package. Postal workers, in general, are ROUGH on packages. They will drop them, throw them, perhaps even play football with them.
You don't have to literally do this, just think about it in your head. If it can't survive going down some stairs, it can't survive the postal system.
For envelopes, I put the items in a clean bag (and/or bubblewrap if necessary). If the item is delicate or bigger, it's usually better to use a box instead.
Note: Sometimes the postal system does weird things, so even with awesome packaging your item(s) may not make it. This is why insurance is good.
Also, I'm aware that brands like meta use doubled-over shopping bags to ship, but I don't suggest doing this at all unless you're willing to compensate someone if something happens to the goods (which is far more likely when using something like that).