First of all, you will need to buy a parcel and a label , if you don't have them yet. Make sure that the parcel is not stamped - if it is, you won't be able to send it again. One parcel can only be sent once. You can buy parcels and labels in post offices and those are usually located in depos (but not in all cities).
Write the player name on the label and put the label inside the parcel. Make sure that there's no typo in the player name (you can copy the player name to be sure) and that the player name is in the first line.
Once the label and all the items you want to send are inside the parcel, throw the parcel on the blue mailbox, that you can find in most depos and post offices in Tibia:
Once you have done that, the parcel will immediately appear in the receiver's inbox. Note, if the parcel gets stuck on the mailbox, it means that there's either no label in the parcel or the player name is incorrect.