There are frequent mini buses daily to Mohan - international Lao-China border check point direction from Mengla bus station (previously from Southern (mini) bus station which is not working anylonger obviously as now all traffic begins and ends at that new station). You need to buy ticket at the counter first - it will print buses licence number on it and that one you'll have to take. Staff doesn't speak english but if you ask one guy that does and has been around transit points in Mengla for ages, he'll charge comission.
It takes roughly 1,5 hour driving to get to Mohan and there you'll need to take Xixuanbanna-style tuk-tuk to the border with Laos. It's nice drive and there are some really nice Dai villages by the route.
If you stay in Mengla overnight you may be surprised of the abundance of love 'shops' in the main street. It feels like being in the red light district, every that kind of place is by the main road, next to hotels, one next to another. Girls are sitting on the sofa waiting for cosumers to get inside while some other girls stand on the street in sexy outfit. If you decide to take their service use condom. They look very young, perhaps many of them are teens lured into sex market and HIV is on rise in the region. It's kinda wierd to see prostitution that opnely on the streets in China but it is becomming more obvious that frontier areas are black hole for sex tourism, child prostitution, drugs and weapon business for the local and regional elites. One can only wonder where is the limit.
There are no ATMs in Mengla which accept foreign cards and if you arrive there on weekends (i.e. from Laos) when banks are closed you'll have to change money on the black market. Usually there is one guy waiting for new costumers right at the regional bus station in the center and he may approach you for his exchange 'service'. Be updated for recent rates if you don't want to be cheated too much (or buy some yuan already in Laos - it's easily obtained regularly through banks or black markets in Oudomxay or LNT).