Ok, so we know what Areas and Zones are, great :D
Now let's fill the level with some useful stuff, like Tiles and Sprites.
Both Tiles and Sprites have similar functions: they get placed somewhere in the level and act like they are configured to.
The only difference is that Tiles have no special functions, they are just here to create the environment of the Level.
First configure your Area settings before going on.
Layers will decide what Tiles/Sprites get place before or after other Tiles/Sprites. You can for example create a bush in the Background or a Tree in the Foreground. The central Layer is Layer 1. Everything in Layer 0 will appear in front of it, and everything in layer 2 will appear behind it.
It is very important to know that Tiles can only interact with the Sprites if they are on the Same layer. Since all Sprites are in Layer 1 (at least most of them), you will need to place your actual level in this layer!
What would happen if you don't? Well, the Sprite and Tile wouldn't collide at all! Take for example you placed a ?-block in layer 0. If Mario were to jump to it from below, he would simply jump through it (as he's actually behind the block). If you want to make sure he hits the block, place it in Layer 1!
Tiles are there to Construct your level. Unlike Sprites, they cannot be configured and will not actually do anything on their own.
To place tiles, select the one you want to place in the Palette on the right. Be sure you Layer is set correctly before placing the Tile.
You can place Tiles with the right mouse Button. If you hold the button and drag the mouse, you can make bigger areas of tiles.
You can move the Tiles with the Left mouse button. You can also use the small box that appears on the bottom right to change the size of your tile area.
Just keep going until you made what you wanted. You can activate the Raster if you need it.
If you created all needed Tiles, be sure to put them into a Zone. Make a little more tiles that the Zone contains for the case that the screen goes too far (not every TV is the same ;) ).
Sprites are the interactive objects in the game. You will find Enemies, End Flags, Star Coins, ... in this category.
The most notable difference with Tiles is that most Sprites can be configured (you can access the settings of the Sprite by clicking it).
Access the Sprites with the 5th tab of the Palette (the rocket). You will see some different categories (use the drop-down box to change).
Maybe you will have trouble finding some sprites you want at first, but you'll get used to it ;)
You can place Sprites the same way as Tiles (using the right button). You cannot create Areas of Sprites, so it will be one at a time...
Note: All sprites Default on Layer 1 (even though they will not disappear when making Layer 1 invisible).