The plug should dome outwards. After the old one is removed and the hole is cleaned out, put some sealant around the lip and place it in the hole. There is a ridge inside which stops it from going in further. Now place a hammer against the plug and smack the hammer with another one. This will deform the center a bit and expand the outside diameter which will cause it to be secured in the bore. Here are some photos from when I replaced mine. I did it when I had the toe board cut out.
http://rides.webshots.com/photo/1316417356041873203xTDshC

Last edited by truckernix; 01/19/2008 11:20 PM.
1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!