The only one I have experience with is the Clickety Click. It's well made,and lasts a very long time.
You just put it on the back of your top bow limb. There's a string hooked to a little chain on it.
To set it is kind of trail and error. The string can just be tied around your bowstring for now,because you will need to adjust it.
Get an arrow long enough for your draw,with a little extra length. Draw the arrow to your anchor point,and have some one mark the arrow where it stops on the rest closest to you(the back side of the rest).
If the clicker clicked before you reached full draw,the string is too short.If it doesn't click at all it's too long.
Once you have it perfect,measure the string on the clicker between the limb and your bowstring.Mark the measurement down in your bow notebook for future use.
Make a mark on the clicker string.
Also make a mark on your bowstring where the clicker will tie in.
Now you can untie the string,and unstring your bow. You want to weave the clicker string in between a couple of strands of bow string.
Restring your bow check you clicker again,readjust if you have too,and you're ready to shoot.