Press X for world map.
Select the gathering hall.
Top option for Gathering Hall (online)
First two options are local, last four are related to online in order of: Create hall, search for hall, join from friends list, enter gathering hall ID.
Create gathering hall options are in order of:
Switch/3DS version exclusion (I think)
Gathering hall name
Monster/quest type target
Quest rank
HR min
HR max
Max number of players
Allow players to join who aren't within HR min/max (I think)
Set password
On the second page are four options for messages describing the kind of room you want to run. One to note in particular is the fourth one on the second page starting with フレンド which means "hunting with friends". Other useful ones are on the fourth page, the first option starting with ホスト says "host picks quests", next is 順番 which means turns I believe, and then 同じ which means farming one monster.
One quirk of the online in this game as in other MH games is you might have to try searching for a room to get the game to connect to the internet and start showing your friends' status properly.
Let me know if I fucked anything up, that should all be correct but my Japanese skills are an incredible mishmash from a bunch of different learning methods.