One of the best ways to improve on iRacing is to race against someone who is faster than you. To watch them and to follow their line... the best way to do that is to race against their "ghost".
Every time you download a Driver or Elite setup from the Majors Garage you get an .olap and a .blap file from the crew chief. These are the files that allow you ghost race against them in a offline practice session.
But how do you use them? It's pretty simple, but there are a few steps, so let's walk through them.
Step 1 - Download the setup .zip file, inside locate the files that have the .olap and .blap extensions (normally there will be one of each).
Step 2 - Open your iRacing folder (in Documents by default) and then find the "lapfiles" folder. (see below image)
Step 3 - Find the track you have the lap files for.
Step 4 - Open the track folder and drag and drop the olap / blap files into it. Don't change the names, just drag and drop.
Step 5 - Start a test session in iRacing. In the Sim select the "Options" tab and then the "Options" tab again.
Step 6 - Under "split time delta" select either "Comparison Sector" or "Comparison Lap" (sector is much preferred for any road course).
Step 7 - Select "load" and you will now see a menu of all the ghost lap files you can race against. Choose the one from your crew chief that you just added.
Step 8 - click the big green "Open" button and start testing. You are now racing against the crew chief. Hope that helps!