Get a wall paper scoring tool while you are there.
Use the scoring tool on the tar paper surface to perforate it a bit. This will let the steam penetrate the tar paper a bit better.
Set up the steamer with the larger applicator on the end of the hose. I filled the reservoir with hot tap water and added a few teaspoons of TSP.
Plug it in and let it start steaming (takes ten minutes or so). Put the applicator face down on a patch of tar paper-covered floor and let it sit there for 3-5 minutes. Move it to a new spot and using a broad putty knife scrape up the now liquified tar paper/mastic combo from the first spot. You will want to experiment with the amount of time you steam an area to get the easiest scraping results.