Routing certain networks over VPN only

Hi, I’m fairly new to routerOS after purchasing a new 4011… I must say, the learning curve is a little more than I expected, but I’m enjoying it for the most part…

My current set up is

ISP > routerOS ether1 (WAN)
LAN network = 192.168.88.1/24

I’m looking at creating another LAN, say, 192.168.70.1/24
Also, making a VPN connection from routerOS to a VPN server using Open VPN.

Now - I want all traffic from 192.168.88.1/24 to go over my ISP’s WAN connection, while traffic to and from 192.168.70.1/24 to go through the VPN only…

Furthermore, I want to completely block traffic between the LANs.. effectively I want the 192.168.70.1/24 network completely isolated with the only network it can route to/from being the VPN connection…

Can someone kindly point me in the right direct or where to get started?

Cheers