You'll need to download a level editor called Mappy. It's a free editor created by some guy (whose name I can't find anywhere on his website).
Once you have that, open and extract it somewhere. If you've never worked with zip files before, you should download a zip utility program such as WinZip or 7zip, both of which offer free downloads (WinZip is easier to use, but advanced users should try 7zip instead). It's technically possible to create zip files on Windows XP (and maybe on Windows Vista) without downloading any additional software, but I prefer these programs personally. Once downloaded, take a few minutes and figure out how to extract the files and run Mappy.
Once Mappy is running, go to File->Open. Browse to your Super Orbulite World folder, and into the "customlevels" subfolder. Choose "custom level template.FMP" to load the custom level template. Actually, I'd strongly recommend that you instead create a copy of the "custom level template.FMP" file and load your copy instead, so that you always have a fresh template any time you want to start a new level.
Once the level has been loaded, you should see something like this:
Mappy with the custom level template loaded
Notice that the editor is divided into two panes. The left pane is the level you're editing, and the right pane is full of tiles that you can place around the level. To place a tile, simply select the tile and then click where you'd like to place it in the level. This is all pretty easy to figure out.
Before you dig into the level editor too deeply, you should make your life a lot easier by dragging the right pane (the tile pane) to the left to expand it until there are six tiles per row, as shown in the screenshot below (the reason is that the tiles will line up better this way):
About the three layers
Click the "Layer" menu once. Layer 0 should be selected (if it's not, go ahead and select it). You'll see that there are three layers you can use: layer 0, layer 1, and layer 2. It's very important to know which layer you're currently editing, as each of the three layers has a different purpose:
layer 0 - your general terrain/background/scenery layer. This is where you place the ground, platforms, and scenery (clouds/trees/grass/rocks). This is also the only layer where tiles can be solid.
layer 1 - enemies, objects (coin blocks and power-up blocks), the player's starting point, and the goal go here.
layer 2 - this is an extra scenery layer that is drawn "below" layer 0, and this is where you place your water. Note that objects cannot be solid here (only in layer 0).
So there you have it. For most of the level sculpting, you edit layer 0. For adding monsters, coin/power-up blocks, and for changing the player and goal's positions, you use layer 1. Then, layer 2 is mainly only used for adding water.
TIP: If you want to try and do something crazy, you can edit specific tiles to be solid or not by double-clicking them in the tile pane. You'll see a bunch of "user data" values you can set; user 1 is the interesting one-- setting it to 1 makes a tile solid, while 0 makes it not solid.
To save your new level, go to File->Save As and save it somewhere in your Super Orbulite World "customlevels" folder. You can then play it by running Super Orbulite World and choosing it from the "Play custom levels" menu on the title screen.
Submitting your finished level for all to play
When you've finished your masterpiece, you can submit your level and have other people play your level. However, it may be a good idea to wait until you create a more advanced level with custom backgrounds/foregrounds/music/settings etc. I'll teach you how to do all this in a later tutorial, so keep reading. :)
Next tutorial:
Tutorial 2: Custom tileset graphics