i dont have a car so im gonna give directions from touhoku school of art and design (geikoudai) BY BIKE (cars not allowed). that means that there is a pretty sustantial road ride to the trail head. depending on your fitness, this could be a day ride.
behind geikoudai is a small park. follow the cobblestones up through the park until it turns into a paved narrow road through the woods. follow about 3 kilos. as you near the top of this road (step 1 of 4) you will see some gazebos and ponds. follow until the road Ts and there is a beautiful expanse of grassy park on your left, a sort of amphitheater on your right and a steep wooded ridge in front of you. turn left and go 1/3 kilo to another T. turn right (going up, not down). go up more to where it takes a 90degree left and goes under a suspension foot bridge. as it starts to go down the other side, take your first right (maybe 30 meters). follow the road up to the radio tower station. when the road deadends, follow the little trail that continues in the same dir. stay on this although its overgrown. it puts you onto a road next to a big pond. follow the road around the pond (to the right) until it Ts (if there is a flower park in front of you at the T, you are doing great).
(step 2)turn right and follow to the tollbooth (30yen for bikes) follow this road for about 5 kilometers. it will T at the road to Zao onsen. turn left and go another 100-150 meters. look for the massive sign/map on your left. this is the trailhead (see map below).
(step 3 [fun step])from the trailhead, you follow the trail up 8 meters to a T and turn left. this trail is not a downhill trail. it is rolling singletrack that is physically somewhat draining (thus intermediate rated) but technically pretty easy. i have seen bears on this trail so a bear bell is HIGHLY rec'd. very soon, the trail meets a gravel road. follow it for only 10 meters (before it turns right) and drop back onto the sngltrk to your left. follow and have fun. lots of different forest types on this trail. beautiful. you eventually come to another road. turn right and follow it til it deadends and the sngltrk continues out the back side. keep going (all told maybe 1/2 hour or so) until you have a final descent to a small shelter next to a lake. (at this point, see the end of this post for extension info)
(step 4) there is a dirt road next to the shelter. turn right and follow the road til it Ts. turn left on the paved road and follow it (steep, down) to another paved road. EITHER 1)take a left and you will find yourself at the intersection previously mentioned with the flower park OR 2)turn right and by staying on that road, you will eventually end up (10 kilometers maybe [mostly down]) back in yamagata city by the ken-sho...
HOWEVER. there is a little more riding from the lake and the shelter back up where you came out. if interested, go to the "zao - nishizao extension trail" post. your choice.