Hey there,
I'll do it just for fun,it's not like you need help or anything
1-To send a message/mail to someone,you should be in a city post (Dartrong city square post or Chernag Settlement Post) that way you can send mails containing items or money or just write things in them
2-To contact someone you can just write prv[name of the person] and then keep typing to talk to that person privately.If you want to make the conversation public,then you can type to[name of the person] and then write,that's the shortcut of doing these for people out of your reach,if you're in the same location as the person,you know what to do.
3-To put an item in a slot,you should first remove the item(s) in that slot.To do so,first you should go to your backpack,then click on the slot you want to remove elixir from on the left hand side of your screen,then equip the elixir you want to use in your slots,there you go.
4-To get to Collections Interface,you should go to your Backpack,you'll see a few tabs above you character,one on the far right is Collections,click on that and you'll see the collections interface.
You're welcome