I love how most of the time the biggest inspiration for field effects is Reborn
That aside, I would suggest you to simply download reborn additional files and open them with RPGMaker to see how its fields work on the scripts BUT, Reborn was built in an older version (v15 I belive) so if you happen to be using that version its mostly copy-paste and editing a few things
If you are ussing a more up to date vesion I would recommend you download the Gen 8 project and simply looking in the scripts how the gen 6 terrains work.
You can easily do this by going in the script editor, Ctrl + Shift + F to search in the entirity of the scripts and searching "grassy" or "misty" for grassy terrain and misty terrain respectively since those are they IDs. You can do the same with "Electric" and "Psychic" for they terrains but those are also types so they wont be so obvious to find
Afther that is the same, just a matter of copy-pasting and editting to your liking. These terrains however work differently than Reborn's fields in three major ways:
1.- They dont alter the battlefield sprites, they behave more like weather in that at the end of the turn a little animation and a message shows to remind you what field is active, but the field stays the same. There are various tutorials in how to change the sprites but I can only find them for v18 and below, none up to date with v19 essentials yet. It shouldn't be too difficult to do but I haven't done it so I couldn't tell you how to do it exactly, trial and error
2.- They have to be activated by an ability or move, they aren't present in the battle turn 0 by any means. Then again, rain can be from the get go based on some places so you can try to use the same code and convert it to fields. In the topic, they dont transform to other fields by using an especific move but they do change the properties of some moves, so you can easily add a new property in a move to active a new field only if another field is active, like fireblast triggering idk... "burning field" if "grassy terrain" is active or something like that
3.- They are all temporal, like weathers, but again, primal kyogre and groudon can have infinite weather so you can use they triggers and remove the condition of having an specific pokemon on the field
Other than that is just trial and error like I said. Im currently trying to make some custom terrains as well but having Elitte Battle DX adds another layer of complexity, but for vanilla essentials it should be pretty straightforward. If it helps in relic castle there is a tutorial in making those field effects but its for v18 and I don't really know how to update them so idk if they still are relevant
(broken link removed)
Anyway, good luck and sorry I couldn't help you more
