A shift lever is an easy item to install, but sometimes it helps to see one done before you do it yourself. There are a couple of considerations to take into account, and that's why I decided to make this post and video.
The first thing to consider is the toe shifter. If it's installed too low, you won't be able to get your toe under it to shift into a higher gear. Also, if it's too low, it might hit the floor board when you push down on it, causing you not to be able to shift into a lower gear. If you're planning on using your toe to shift into a higher gear, you also need to consider how much movement you have in your ankle as well. If the shift lever is too high, your ankle's range of motion may not be enough to make that shift, leaving you to have to lift your foot off the floorboard to shift fully.
Secondly, the heel shifter. Though you don't have to worry about getting your toe under it if it's installed too low, The shift peg might hit the floorboard and not be able to shift into a higher gear properly. The other consideration on the rear shift lever is to be sure it's high enough so that if you drop your bike, the floorboard won't fold up and hit it, which could damage the lever or the shift peg. Before you put the bolts in to tighten it, lift up on the floorboard to see if the lever is high enough to be unaffected in the event of a fall.
This short video walks you through the process to install a shift lever on a Harley Davidson Touring Motorcycle.