Mandalay is one of the biggest cities in Myanmar.
You should take a public bus to village called Myitnge or Mynt (depends on spelling). Bus usually goes from Zegyo Market (the one with the clock tower) but sometimes you have to go down 84th street to corner of 29th street. Then ask about the bus to Myitnge. It was probably bus number 8 but I'm not sure. It should cost around 500 Kyats.
On the way to Myitnge there's a big roundabout, get off there and turn right. At the beginning of highway there is a big roadside with some stalkers and you can easily hitch there. Lot of car go only to the new Mandalay Airport. You can go with them because airport is very close to the bigger highway which makes also good place to start hitching towards Yangon.