Tricky to describe without a map, but here goes.
Get off the train at Triemli (I think you are allowed to carry on to the next stop with the bike if you want to do even less climbing.)
Head in the same direction as the train and turn left at the end of the platform to cross the tracks.
Follow the road that is in front of you, going up the hill and to the right. Keep following that road for a few km until it comes to a T intersection where the right turn crosses the train tracks and the left turn makes a hairpin back in the direction you came from but more uphill. Take the left turn.
Now keep heading roughly in that direction towards Üetliberg. There is no defined route up now. As long as you keep heading generally south and uphill and don't cross any railway lines, you will reach Üetliberg one way or the other.
When you reach the train station, keep going up the road to the very top.
Now the fun begins. Head back down the way you went up for about a hundred meters. When the road hooks sharply to the left, there is a single track going right between some rocks into a park. Follow the track to the far left side of the little park and back into the forest. This will then take you back down to the Triemli station, and is pretty obvious most of the way down. If you are uncertain, then on a Saturday morning somebody is bound to come past sooner or later who knows the way.
Hope this doesn't get you too lost.