SFXs is short for sound-effects; in other words, the "Bang" "crash" "grab" "glance" etc that you see in manga. SFXs are usually considered the most difficult thing to do, mostly because they're usually placed on top of images (hence, there's lots of cloning involved). Different groups have different requirements regarding SFXs: some require all SFXs to be edited, some let you leave the big ones unedited, and there are some who don't want them edited at all.
We ask that you edit as many SFXs as you can, but you can subtitle the large ones (just write "Sfx: boom" (for example) near the SFX).
Considering you already know how to clone, I'm just going to talk about how to style the SFXs here. It's best to use a variety of SFX fonts. You can find some SFX fonts in the font pack in the Typesetting section, but really, you can get them from any fonts site.
Basic, easy SFXs are the small ones. The most styling you'll want to do on these is to add a stroke or maybe warp the text. Here are three examples:
On the "tap"s above, all I did was rotate them. Always have your sfxs at some sort of an angle.
For the "ha" SFX, I kept it as close to the original as possible. White text, black stroke. To create the black stroke, right click the text layer and chose "Blending Options...". Choose the "Stroke" style, and set the color and size you want. The ideal stroke size is 2-3px.
The final example I want to show you combines multiple strokes with a text warp.
To create that effect I took my "creak" text layer and put an Arc Warped Text effect on it (see this). Then I duplicated that layer, so I had an exact copy right on top of it. On the top layer I added a stroke of 1px, black; on the bottom layer I added a stroke of 3px, white. To look at the actual psd to see this, download it here. Head over to the next section for some more SFXs tips and tricks.