Hi I was planning to create some new tilesets and I need to know what exact guidelines need to be respected in order to create items , walls that can snap into position one with the other aligning perfectly ...

The main thing is just to keep your sizing correct. Since your creating pieces that work together, it's always good to memorize your dimensions. I don't know if you're using 3ds Max, but 3ds Max units directly translate into Skyrim units. So for example, if you have a cube with the dimensions of 1024x1024x1024 on all sides, then it will take up the same amount of unit space in Skyrim. So just figure out what unit you want to work around and base everything off that. So if you create a wall piece that's 1024x1024 units at it's base, and 4096 units at it's height, then you'll want to make all the other pieces match those units so they'll "snap". Another suggestion, and then is something I personally do, but I always put my set pieces in the center of the scene, so that the pivot point is always centered for the piece at 0,0,0. Something I just like to do as it generally makes rotating pieces easier as well.

They always snap in powers of 128. Meaning they snap at 128 units, 256/512/1024/2048, etc.

Just pick one based on what you're making and make it that size. 128 is for VERY small things, like the wasteland homes in Fallout. 256 is the standard size, for small hallways and rooms. 512 is for large hallways and rooms. Then I believe the epic pieces for caves are like 2048 or 4096. That's the most important thing.

On top of that, you have to make seperate sides of rooms if you're doing rooms. The top is 1, left is 2, bottom is 3, and right is 4. (If you hit T and look down at it) The designer should always remember this, and can place them quickly and snap them together properly. Of course, if the meeting point of your tiles is always the same on all sides, you don't need this. (Like the Generic tileset where it's 100% flat)

That's what pops into mind right now.