There are 2 possible paths to reach the small sanctuary on 5th level, the fast but a lot less interesting way and the way via the footbridges that go up gradually (at the first branch with 2 stairs, choose right staircase). The views over the valley are superb and you will also see a sanctuary on a peak. The last of the 7 levels is normally now closed to the public as it is infested with ... snakes. Count 2 hours to go up and down and enjoy the mountain.
The temple can be visited from Udon Thani or Nong Khai but you will have at least 3 hours drive to reach it. I suggest you come to Bueng Khan, capital of the province where the Wat Phu Tok is located and spend a night there - you will find here a list of accommodations in Bueng Khan - and see with them to rent a car with driver to take you there or rent a scooter to go by yourself. Otherwise near the clock tower there are always tuk-tuks but the drivers speak little English. Count around 1.000 THB for round trip with 2 hour wait on site. If you are a bit more adventurous, you can take the bus 225 still at the clock tower up to Si Wilai (20 THB, 45 minutes) and then a tuk-tuk to Wat Phu Tok (around 300 THB round trip).
To get to Bueng Khan from Nong Khai, at the city bus terminal, you can choose between local bus (3h - 150 THB) or minivans. From Udon Thani, it will necessarily be a minivan.
Wat Phu Tok is a temple and when you visit a temple in Thailand there are some rules to know and follow :