It's easy just assemble any blocks in the same order on both side then hit it was a flint on both sides and your done! Then all you do is walk through and your there!
You can dress them up to your liking as long as the block order is the exact same on both sides it works fine!
https://www.spigotmc.org/resources/wormholes-portals-you-can-see-through.43188/
