I think this guide here explain it best and contains important quests
https://www.wowhead.com/guides/garrisons/guide-to-garrisons-in-warlords-of-draenor
https://www.wowhead.com/guides/garrisons/quests-to-unlock-a-level-1-and-level-2-garrison
best to take a look at one of the existing guide and copy it from example eg if you open the file
you will see first couple of lines are
A is for Accept steps
C is for Complete steps
T is for Turn in steps
Then you need to add the questid tag |QID|…| , this can be found in wowhead URL link eg
https://www.wowhead.com/quest=34575/step-three-prophet the quest id is 34575 for that quest.
Then add the note tag |N|…| where you can add any text and and coordinates, in this section you can add special tags like
(npc:…) will automatically convert to the correct localize NPC names the npc id can be found from wowhead links like quest id
other special links are (qid:…) (item:…) (spell:…) where it will need the id number to convert to text and tooltip.
Coordinates links should be formatted as (xx.xx, xx.xx) note the dots and commas, if you add multiple coordinates inside the |N|…| tag then the addon will place and connect all the waypoints in the corresponding order in the guide.
{Valley of Trials} is optional this is the subzone name and the brackets {..} tells the addon that it can be localized and converted to client’s language.
so after |N|…| tag there is also a few things you will need for custom guides
|Z|..| tag which will included the Mapid number inside it so the waypoint is placed in the correct map, see the mapid list attached in this post
|NPC|…| tag, include npc id in this if you decide to use any (npc:…) tag for the model viewer to display.