Suggestion 1: Watch the video tutorial.
Suggestion 2: Play with it for a bit, saving items that you don't care about just so you get a feel for the program.

Here's a quick, hopefully not-too technical version of the process:

Load the clothing item by selecting the .cr2 file. (That's the file you see when you go to the library in order to load the figure in Poser.)
Load the character it currently fits.
Load the character you want it to fit.

Click on Autoscale and the program will guestimate how the clothing should align and scale. I've had items that perfectly fit in place. I've had items that I reloaded because I didn't like the autoscale's guess. Usually it gives you a good start on the process.

After that, you select the clothing item (right click, select all) and move and scale it around so it roughly fits the character. You can turn the original figure transparent or the clothing item transparent for better visibility.

The last part is tweaking. You can select polygons or points and move those around. There are a few different selection tools you can use to get the parts you want. (In my opinion, it takes a little experimenting to see when polygons work better vs. points.) You also have the "soft select" option which is like a Poser magnet, but far easier to use. You center the magnet on a polygon or point and move it in a direction. Imagine having a sheet of taffy. Pinch a bit of it and pull. That's soft selection. You can increase the radius of the taffy pull so more of the model is affected.)

When you're done, you save the clothing item. You can select what to call the item's .cr2 file, what to label the new .obj file, and what clothing item to swipe the new .cr2 from. If you take the M3 casual pants, for example, and convert them for use on V3 Male, you might want to use the .cr2 file from the pants in the tunic pack since the tunic pants would move in similar ways to the newly converted casual pants.

One thing that I appreciate is that you can adjust how sensitive the program is to your mouse movements. I'm not very dexterious, so I set the defaults low so it takes a lot of movement to get much change in movement or scaling. It's much easier for me that way.

There is a video tutorial included, though being an impatient geek, I skipped it. I've heard some good comments about it, though.

Cheers!