Basic Process, please keep in mind this is an overview, and that each of the steps can be done in more then one way)

1. Take the OBJ file of the figure for which you want to model the clothes and import it in yor favorite application. (Usually scaled up 100, 200 or 500 times from Poser units).

2. Plan out the piece of clothing (some people make a paper or a digital sketch, some work off of a photo ideas, some just wing it)

3. Model the piece in your favorite Modelling application. Mostly using subdivision modelling, or a combination of subdivision and spline modelling. Most people do it by modeling the piece around the figure exported in step 1.

4. UV map the model.

5. Create textures.

6. Group for Poser 'rigging' and export into an OBJ file (scaled down to a poser scale)

7. Import into Poser, and rig to fit the intended figure.

8. Import any additional morphs, and save as a figure.

Most of the time, there is some back and forth bvetween the steps, and some trial and error. There's a number of programs and utilities theese can be done in. 
If one is working on it 2-4 hours a day, it probably takes about a week to create a medium complexity piece of clothing, with 2-3 morphs.

If anyone else wants to jump in and expand on the basic steps, please do it!!!

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
BadKittehCo Store  BadKittehCo Freebies and product support