Perfect World is the final step, after you have a decent heightmap. And if you can't draw one you can find hightmaps of the real world online, but it's been a while since I looked (finding real tropical islands for use in the first Tropico game) But let's just say you have sorta followed my earlier directions using GIMP and Wilbur.
Open PF. Select the Elevation Map tab. Click the Import Bitmap button. Select your map.
Use the Sea level slider, make some oceans. Use the Tree Line slider to make your mountain ranges as big as you like.
Open the Rainfall Map tab, Click the Generate Rainshadow button. Click the Generate New Noisefield button.
Play with the sliders to get the right balance of forest, grassland, and desert.
Open the Drainage Map tab. Set the filter size to 3. Click the Generate Elevation Standard Deviation Map button.
Play with the sliders for dune/swamp, rocky/grass, and hills to get a good mix.
Open the Volcanism Map tab. Click the Generate Elevation Cusp Map button. Play around with the high and low cusp sliders.
Play around with the Volcanism sliders. Remember, you need some areas of red if you want volcanoes but red means no sedimentary stone in that area.
Unlike the other tabs, it's a good idea to use the Generate New Noise Field button here, unless you want all your volcanoes at the same altitude.
Don't raise the lower cusp too high or you will get a lot of ocean volcanoes.
Open the Savagery Map tab. Choose a savagery map type. Generate the map and noise field, if you want lots of little disconnected regions of savagery, turn up the feature size and the smoothness (they are inverse), if you want the opposite, do the opposite. Play around with the savage/neutral/calm balance sliders.
Almost done.
Import your workld gen file from the file menu. Click on the map parameters tab. Select an island or region of the appropriate size in the param set Name drop down box. Now type in a new name, this will be the name you select in DF world gen. PW will ask if you want to copy parameters or rename, choose copy.
Click all the "Use Perfect World Maps For:" check boxes. Set your parameters:
End year: you know what this does, right?
Mineral scarcity: Higher is scarcer
Mega, Semi-Mega, titans, demons, Bogymen, Vampires, Curses, Rains, etc: Higher is more of these nasty things. But you want that, right? Just don't go crazy. Thousands is crazy, hundreds maybe not.
Good and Evil square counts: Small, in the low hundreds or more. Medium in the mid hundreds or more. Large, do you want an evil or good ocean? Just set these to 1 then.
Peaks, volcanoes as you like it, but you can only get as many volcanoes as you have 100% volcanism squares.
I like setting the erosion cycle count to 300-350.
River starts: these are rejection parameters, they don't actually influence the number of rivers. You probably won't need to change them
Other settings: well, it's your world.
Now look at the Map Generation parameters sub-tab. If you are using the PF maps, you won't have to set any of these except temperature. Set that as you like it. Oh, if you turned an island region into a continent, the number of edge oceans may be wrong, and that's a rejection parameter so it might keep your world from genning. Just set it to 0 and you won't get rejections.
From the file menu, export your map. QUIT DF if you have it running. Start it and go to the advanced gen screen, pick your region and generate.