Found my cue sheet.

Quick way is Sharpsburg Pike / route 65 south to Sharpsbrug.

The pretty way:
Sharpsburg Pike / route 65 for about a mile south of I70
L at Poffenberger
1.7 R at Garis Shop (before US40A)
3.6 becomes Barnes
1.6 L on MD 68 at T
0.3 R on Mill Point (hill)
0.9 L on Monroe
1.8 L on MD 34 at T
1.1 R on US 40A at tl (Boonsboro)
0.7 R on MD 67
1.8 R on Mt Carmel Rd at chruch
0.3 R on Dog Street
1.8 L on S. Main at T
cross MD 34 to Keedysville Rd
1.0 L on Mansfield (after 2nd stone bridge)
now your are in the battlefield,
1.8 L on Smoketown at T
0.7 L on Dunker Church
visitor center on the left, pick up map if you want to explore
(ps they will charge you to use the bathrooms)
0.3 L on MD 65
Battleview Market on L is a good stop for food, toilets are free.
continue on MD 65 into Sharpsburg
R on Main St at stop
R on North Mechanic becomes Mondell
2.7 bear L at Y (narrow and twisty)
0.3 bear L Bowie Rd becomes Taylors Landing
0.7 R on Tommytown (hills)
0.5 R to stay on Tommytown
1.0 L on Bakersville @ T turns into Spielman
3.5 R on Downsville Pike @ stop
3.5 R on Rench Rd
2.1 L on MD 65 / Sharpsburg Pike and back to Hagerstown

About 36 miles total from I70.
Explore the battlefield if you have time.
To save some climbing, don't go down Mondell to the river. Go back on MD 65 to Bakersville Rd.

Have fun.