Take the "A95 Autobahn" (freeway) from Munich to Garmisch-Partenkirchen and then Grainau.
Or take the B17 road from Augsburg to Schongau and continue on the B23
in the direction of Oberammergau and Garmisch-Partenkirchen and then Grainau.
Or take the Ulm-Kempten "A7 Autobahn" (freeway) south towards Innsbruck, Austria; continue towards Reutte and Ehrwald, Austria; then, just before Ehrwald, turn left toward Grainau and Garmisch-Partenkirchen.
If you are coming from the south, you can reach Grainau via the Brenner. Drive toward Innsbruck, and then past Innsbruck toward Zirl, Seefeld, Mittenwald and Garmisch-Partenkirchen, then to Grainau.