Boy, this site sure has been useful to me in finding cool ROM hacks. The DSvania Editor is a truly awesome tool. I was able to make some hacks for POR and OOE.
Thanks, I'm glad you're enjoying it!
However, all I have done is to add more enemies, and more abilities for player characters using the Player Editor. What I really want to do now is to edit backgrounds. I have tried adding the tall trees from Ruvas Forest into the Castle Entrance hall. Since the background behind the windows shows only a boring gray sky, I thought I would spruce it up by adding a forest background to it. However, when I attempt to use both Tiled and the editor for it the trees always turn into jumbled copies of the hallway sprites.
Please excuse my complete noobness. I really would love to make the castle look more interesting in the game and to add a couple more forests into the game (I just love forest stages). I just have no idea how to add things to the background (like the example above). Please forgive me if this question has already been answered and if I am being inconvenient at all. I would appreciate it if anyone could please give me some clear instructions in what to do or to know what exactly am I doing wrong. Also, I am really not well-versed in coding language at all, so please excuse my ignorance. The only reason I was able to get as far as I did with the Editor is because of a simple drag and drop.
You can only use tilesets from the sector you're in - each sector loads in all the tilesets it will need when you enter it, so using tilesets from a different sector is impossible since they're not loaded anywhere. Tiled doesn't know about this technical limitation of the DSVania game engine so it lets you use as many tilesets as you want, but when you import there's no way for DSVEdit to handle the other tilesets so it corrupts the room's tiles.
You would need to replace some part of the Entrance tileset GFX with the trees from Ruvas, then you can add them in.
Also, I would love to know how to import and export both monsters and background sprites between each DSvania (like putting Slogra in OOE, and the library from OOE into POR). Is there a place where these instructions are at, so I can go there and read them?
You can export and import GFX with the GFX editor, as bogaabogaa mentioned. You can look in the readme for the section called "How to edit GFX and palettes" for more information.
It should also be possible to export and import sprites across games, though I've never tried it myself. The "How to edit sprites" section of the readme explains how to export and import them.
But note that in both cases, you can't add new GFX/sprites. So you'll simply be replacing existing backgrounds/monster sprites with ones from a different game.
But when you say "export monsters" - you can't export their AI between different games. That would be extremely complicated. It's theoretically possible, but no one has managed it yet as far as I'm aware. So if you wanted Slogra in OoE, you could make something like say a Merman look like Slogra, but it would still act like a Merman.