Apparently it is not so straight forward. But after doing it once it is quite easy.
1. Get the latest version of Aia and Extras from AMP. Open the Aia file.
2. Right click Extras file and click "Import into maya"
3. The Extras control will appear to the left side of the feet.
5. Change "Body Selectable" to "on".
6. Select the Clothes, shoes or Hair to change. And press delete!
7. Select through each Extras control Letter and click the preferred "show".
H-Hair T-Top B-Bottom S-Shoes A-Accessories
8. Select the Top.
* See updates on "Wrapping clothes to Skin" at the bottom of the post before doing this step.
9. Shift select the body.
(For Rigs that cannot select the body by clicking like Bishop, go to outliner and ctrl select the body geometry.)
10. Click Wrap. Repeat for Bottom, Shoes and Accessories. For the Hair go to the next step
* See updates on "Wrap" at the bottom of the post before doing this step.
11. Select Head Curve.
12. Shift select Hair Curve or Hair geometry.
13. Click Parent constrain.
(Basically anything i want it to deform when the geometry is bent or stretched i wrap, anything i only want it to move together but not deform i constrain. Example: For Bishop, top, bottom, shoes, eye brows - Wrap. Hair, ears - Constrain. Accessaries - Depends how i want it to be.)
14. For clean-up and also to reduce the Rig file size, delete everything that is not needed. Including the Extras Control and all the Extras that are hidden.
*Last note if the Customised Rig becomes very lag when moving the body around, go back to the original Rig file and set the geometry to Low quality display. Select the geometry and press 1 - Low quality display, 2 - Medium quality display, 3 - High quality display.
Updates (28/1/2016)
Wrapping clothes to "Skin"
A new trick found by Justin Owens from Animation Mentor. If the clothes is wrapped to the body geometry when it is at Low Resoution, when switching to High Resolution the clothes will not work. That is because when selecting the body geometry during the wrap, the current visible body geometry selected is under "aia_anim_geo_grp". In order for the clothes to work in both High and Low Resolution, the clothes have to be wrapped to the body geometry under "aia_skin_geo_grp" which is hidden. Follow steps below.
*If this is too complicated to do and there is no need to switch between High and Low Resolution then skip this method.
1. Open outliner
2. Select clothes
3. Find "aia_skin_body_geo" and ctrl select it
4. Click option for "Wrap"
5. Click "Create". (Make sure "Exclusive bind" ticked)
1. Click "Wrap"
2. Make sure "Exclusive bind" is turned on.
3. Click "Create"