Step 1: stand on the passenger side and/or the front of the car, reach around to the back of the intake manifold with your left hand
Step 2: locate MAP sensor electrical harness clip. It goes left to right (wire on passenger side, sensor on drivers side).
Step 3: unclip MAP sensor harness by lifting the clip tab and pulling left
Step 4: locate the MAP sensor and the retainer clip holding it in (by feel). the sensor is pushed down in an adapter in the intake manifold
Step 5: push the clip towards the back of the car and pull up/down on the MAP sensor. try not to break anything (like the vacuum port right below it) in the process. It'll be a little difficult. If you can't remove it that way, you need to pull the intake manifold. There are write-ups on LS1howto
Installation is reverse of removal. Here's a pic to help visualize:
Oh yeah, and wear a longsleeve shirt and/or jacket if it fits. It's pretty easy to scratch up your arm on the cowl.