The Doi Tung viewpoint is tucked in behind the expansive arboretum that grows new trees and plants for the Royal Villa and is about 9 km away from the villa itself. Some tours go to the arboretum, but you probably need to be self-powered to get to the viewpoint. From the Arboreturm, continue past (which is well signposted from the villa) about 300m and follow the signs to Mae Sai, you'll see a set of clay steps going up to a wooden pallisade wall. If you go past it you'll run into a small border post about 100m past the viewpoint.  You can also access this attraction from Mae Sai.  From Mae Sai, follow the one way system West and up on the mountain ridge.  You will pass at least one army checkpoint who will probably stop you and ask you where you are going and why, this is a great way to head back from Mae Sai/Tachileik and take in the Doi Tung Royal Villa/Arboretum on the wat back (or vice-versa).

There is still occasional drug running along this border since there is no natural landmark  to mark the border with Burma, e.g. a river, simply the mountain ridge itself and Thai farmers literally run shoulders with their Burmese counterparts, hence this fortified Viewpoint and the numerous army checkpoints you may pass along the way.

When you climb the stairs, you'll find gun emplacements, trenches, pillboxes, a small memorial and a small army camp where soldiers of the Thai army keep an eye on the Burmese. There are great views of Burma from the hilltop, which is right on the border itself, and you can see a couple of very basic mud-hut villages in Burma which are quite amazing.

There is even free tea and coffee, but please leave a small donation to help the soldiers to pay for more.