Entering the North of Laos via land from Thailand is pretty simple.
From Chang Mai a bus will take you to the border. If you so desire, on the way you can stop by the famous Chang Rai’s White Temple for a couple of hours.
This is more than enough time to visit the temple, a very crowded place, and jump on another bus to reach the border with Laos on the same day. No need to linger in Chang Rai really.
Once at the border, in Chang Khong, you will be able to take care of formalities and cross the bridge over the Mekong river by foot.
On the other side, some locals provide transportation for your next destination and accept Thai Baath as well. The currency is stronger than Laotian Kip.
Take some passport size photographs with you and some cash to pay for your visa.
If you do it the other way around, coming from Laos to enter Thailand, then the Laotian town to be is Huauxai.
That’s it. Easy peasy, and relatively quick!