Using Custom Sprites???

I have created some custom trees. I would like to use them in addition to all existing sprites. However, I am only able to replace an existing Sprite (using XWE). Basically my question is just how to insert a custom sprite. I've been reading about the SS_START/END tags - any more detailed info about how to place me new sprite within them?

In addition, when I was able to successfully replace an existing sprite using XWE it was only in the DOOM2 IWAD. How would I add a sprite so that it is "distributable" in my PWAD?

Just trying to knock off the rust and learn the newer tools. Doom Builder is one fine editor.

Is this for any engine in particular, or for any whatsoever?

Using the standard engine there are two ways to apply sprite replacements:

One is to name them as the original (TROO* if you replace the Imp, for example), apply the SS_ markers around them (SS_START at the top, and SS_END at the bottom). The user will have to patch the wad with the rest of the sprites from the IWAD before playing, unless he's using a more modern engine which can load sprite replacements directly (any of the currently maintained community ports and engines can do so).

The other way is to rename the sprite replaced through DeHackEd's F7 (let's say TROO* to TRO1*), and then give that new name to the sprites you add, placing them between SS_START and S_END (only one S on the ending marker). The user would have to apply the DeHackEd patch to the engine, though you can also place the patch in the wad (as a lump called DEHACKED), so that modern engines can load the stuff directly.