I honestly felt like this was easy to figure out, but I guess not? I'm always having to answer "How do I send things via feathermail?????????" so here this is. enjoy.
First, go to the GH (Or any other place with an owl) and walk up to Hedwig (an owl).
Second, Click the owl (without item in hand if its a book)
Third, Click the book and quill which is named "Send Item (#0386)"
Fourth, get the item in your hand
Fifth, Say your target's IGN in chat.
Sixth, Your item has been sent! When your target checks their inbox, it will have the item.
We will be sending an item to Raven Maxime, who has the IGN "Dumbledxre".
Sending it (POV of Sender, Willow Malamius.)
Recieving it (POV of Reciever, Raven Maxime.)
I hope this helped! Leave any questions below. If anything changes, the post will be edited. If I fail to edit, post changes below.