The best and easiest option is to start at the on-ramp of the B207 in the very west of Ratzeburg, next to the McDonalds and close to the train station. Towards Hamburg it's probably best to hitchhike south first to get to the on-ramp of the A24.
Try to get to the on-ramp of the A24 or A20 first via the B207 as described above depending on where you need to go. For hitchhiking towards Schwerin it is probably better to hitch rides along the B208 and B104 starting somewhere in the east of Ratzeburg.
Very easy from exit #3 at the A20 called Groß-Sarau as there is a roundabout with all the traffic coming from Lübeck.