Nine times out of ten, you'll be using this procedure:

 

Load the base figure and all of its morphs.  Assume Vickie 4 here.

Make sure the base figure is selected.

Load in the clothing prop.  Load in its morph poses, if any.

Click the clothing prop, make sure its body is selected.

Apply one of the Vickie 4 magnetize clothing poses.  There's more than one pose because you have to use a different magenetize pose for each piece of clothing.

Then conform the clothing to the base figure.

That should do it.