to get the portal ingame all you do is:
Go into map maker. Click and load your map up.
On the first Tab thats named 'Map' look down to find 'Has Portal' and Tick that box.
The x,y,z and arrival yaw are for inside the map.
When you've jumped inside your map you'll spawn at a random place. Run to where you want to spawn in that map. So the next time you or someone jumps into that map they should spawn where you've placed the code.
Actually, that is a different issue that I was not referring to here, but I was struggling with that, thanks!
Its best that you use your own heightmap's that you've made yourself or you can request one to be made for you. Its best to not keep using Watermills or default heightmaps. Not everyone will download a map that resembles watermill with just different terrain textures and objects. People Like to explore something they havent seen. However it is good to practice with them to help you get better. So if you don't feel like downloading gimp or photoshop just ask someone to make a heightmap for you.
And yes, I was using this map just for practice. It will be a long time before I encourage someone to join a map I make, right now I am just making sure I have the basics down, so when I get a good idea, I can apply it.
Also, from this comment, you seem to be implying that there is no way to get around photoshop or gimp? I have the default "paint" program, is that good enough, or are there necessary options in photoshop and gimp that make anything else out of the question?