First, there are two sizes of tenons. The older 1911s had the skinny tenons and the newer guns use the wide ones. Make sure you buy the right size, probably a wide one.
Disassemble the slide and observe the little rectangle directly under the sight. Place the slide on the edge of a wood block. The sight will be on the bottom and hanging over the edge of the block. Use a punch and drive straight down on the dead center of the rectangle. The first few hits should reveal the "rivet head". Drive the sight all the way out.
Inspect the inside of the slide. You will notice a recessed area where the rivet head goes. Clean it out as best you can. OK here's the tricky part. You will need an off set punch with a bevel tip. Look at
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=916&title=1911+AUTO+FRONT+SIGHT+STAKER They cost about $30. Place the new sight in the tenon hole and seat it all the way in. Place the slide on an anvil or surface where you can rest the front sight on a solid metal plate. With the staking tool and a hammer, use a series of straight down light hits to form a rivet head. Continue until the sight is securely bottomed out and tight. Take a half round file and dress the rivet head until the bushing fits without hitting the tenon. If you get wild and take to much head off, the sight won't stay on. If you try to use a straight punch, you can't get a straight down hit. The angle hit may damage the slide and won't secure the rivet.
I've done a good many of these and I'll say the first few are not that easy. Once you get the hang of it, you can do one in 10 minutes.
