The first thing: are you sure the problem is a bad igniter? Did you check the igniter for continuity?
About the idler pulley: yes, it just clips in.
After you placed the idler pulley, make a loop on the belt, put it through the idler pulley (under the roller) and around the motor pulley. The belt configuration picture is attached.
- Here are the break down diagrams for the Whirlpool dryer
Model LGR7646EQ0
Gene.