Above is a quick sample of what a common extruded shape would look like. I actually built this with the old KPT Vector Effects Xtra in FreeHand 8, which took about 2 minutes to accomplish.

Since that Xtra is unavailable, I'll be showing you below how to mimic this same effect by hand using the Perspective Grid as your guide.

1) First, we'll start with a simple group of shapes applied to a 1 point Perspective Grid. Above, is some text that I first converted to paths, then applied to the grid.

It's best to start with something simple like this to get the general idea of how to accomplish the extrusion. Then go for more complex shapes later.

You can scale the object when it's on the grid by keeping the mouse button pressed down and using the number keys 1-6.

2) With the Perspective Tool selected, position the cursor over the grid. Then while holding the Shift + Option (Alt) keys down, click and drag the grid over to the side.

This will clone the perspective shape and move it constrained on the grid. Watch the animation above to see it in action.

3) When you get something you like, hide the Perspective Grid.

Then select both the shapes and go under the View menu -> Perspective Grid -> Release with Perspective.

This will release both shapes from the Grid keeping the perspective appearance.

4) Now to make things a bit easier to work with, create a couple of new layers like shown above.

Put the front shapes on the "Front Faces" layer and the back shapes on the "Back Faces" layer. You'll be creating the extrusion shapes on the "Extrusion" layer.

4) When building the extrude shapes, it's much easier to work in Keyline view, so turn that on. Also go under the View menu and turn on Snap to Point.

With the either the Bezigon or Pen tool, start with the back-most shape you are going to add the extrusion to.

Watch the animation above closely as I start adding extrude shapes. You can see that I'm creating polygons between the front and back shapes that will mimic the look of an extrusion. If you start getting confused with the lines, switch back to Preview view mode to see how things are starting to look.

5) You really only need to build the extrude shapes that will actually be seen when finished. So don't waste your time building the shapes at the back.

Above you can see all the shapes needed for the letter H. If you find that some of your shapes overlap others, just remember that you can use the Move Forward or Backward commands under the Modify -> Arrange menu.

6) After you've built the first extrusion, give the new shapes some fills to add to the realism or depth of the shapes. Above I used some Gradient fills from tints of the first blue color I started with. Then added some Black for the darker colors that appear on the bottom.

6) I decided to change the color of the F to green for something a bit different.

Then I repeated step 4 to add the extruded shapes. As you can see, there were very few shapes needed for this particular shape.

And finally, I finished this extrusion off by filling in the F with similar gradient fills like shown above.

Very simple, a bit laborious (depending on the shapes), but effective.