There is parking and restrooms at Morley Field.
The official trail map indicates that this route be done counterclockwise, but it will be safer and more fun to do clockwise. It is very poorly marked, and doesn't really seem like much of a trail than a route one can take through the park.
Begin at Morley Field Gateway. Head north to Upas Street and turn right (east). Use the road or go along the grass lawn, up the hill. At Pershing Drive, turn southwest. You may choose to use the bike lane from the intersection (it continues down the entire road and is always an option). If you keep to the shoulder, you'll have to hop onto the road past the disc golf course. At Frisbee Place, you may cross the road and take the dirt path on the shoulder between the street and the golf course if you wish. Continue downhill.
You'll have to cross the street again -- a good place to do that is where the road to the Velodrome comes off to the right (there is a sign). You may also choose to take this road back to Morley Field. The trail map indicates that you stick to the road shoulder, but there is a wider path a few feet to the north. The road shoulder here may have a lot of brush and weeds, depending on time of year. As Pershing Drive turns south and begins to descend more steeply, stick to the road shoulder. Until the bottom of the hill, this path is narrow, with some rocks, a couple wooden bridges, and a steep drop-off to the right.
Either turn right onto Florida Street and use the bike lane, or instead you may leave Trail #13 and continue on singletrack along Balboa Park Trail #22
northward. Remember, bikes should use the bike lane on the right, with traffic, and pedestrians should use the lane on the left, facing oncoming traffic. Go all the way up Florida Street to Upas Street and turn right (east). There is a steep hill here, you may continue on the street or on the shoulder. Take the driveway into the parking area, and return to the start.