Coop > Raid > Create a Room.
For low level raids (minimum join rank < 101), stay on 'Standard' .
For HL (minimum join rank 101+), select 'Impossible'.
Options:
'Minimum Rank' and 'Player Type' can be left alone.
Set 'Repeating Quest Settings' to the amount of players you wish to have in your train.
Set 'Player Limit' to the maximum number of players that can join the type of raid you are hosting.
Generally you will want to leave 'Player Ready Check' off.
Set your message and posting accordingly.
Refreshing:
Your room will slowly be pushed to the back of the room board if you do not 'refresh' it quickly.
Open the room settings (the 'i' on the right) once it has been created and toggle between 'hide' and 'display' in
the room board settings to boost your train to the top of the list.