from mudahan immigration there is shuttle bus - 50baht will bring across the border / keep the ticket stub & after clearing immigration you can use it to take the next bus to the main bus station where you can find guest house & the consulate. attached is the daily timing from mudahan to savannakhet.
if you haven't utilised visa exempt at all then you can try - 20k baht / 1 passport photo / be at Tesco lotus car park at around 3am onwards daily. the driver will bring you go & back. at bam laem just follow your fellow passengers on what they do
it's thaivisaservice.com. you must be there by 3am nowadays as they leave really early. just go to the onnut tesco lotus carpark & you will see a van / driver just ask & confirm. like i mentioned earlier you must be there early coz they will depart early if they have enough passengers & according to the driver nowadays normally only 1 van / second van will only come if they have the required number otherwise if the you can't take the first one, you probably might have to come back the next day