My router has ip 192.168.1.250 and I have an openvpn server/client at 192.168.1.2 which serves the subnet 10.8.0.0/24. I have a permanent route on mikrotik for that network and generally openvpn works fine
What I want to do is mark packets from/to that network (10.8.0.2) in order to prioritize them later.
Can someone guideline me in general how I can achieve that? What kind of rule should I input for mangle and then for queues?
You could mark the connections based on src and dst, then mark packets based on those connections. You would also need a general bucket rule set to catch all other traffic (or use the no-mark in the queue config). At that point, once they are marked, depending on how you want to handle those packets, change the TOS, DSCP. Then basically write your queues to prioritize based on those mangle rules.
There is a plethora of options to do what you are asking, really it comes down to what you really want to do with the packets..