Hi Guys i currently have a mikrotik set up with a 192.168.0.0/23 range and a 192.168.20.0/24 range i would like only for a specified ip in the 192.168.20.0 range to have access to the 192.168.0.0 range how do i set this up ? at this stage both networks have full access to one another?

IP->Firewall (Forward allow only for these ip's in that desired network)

Expanding on what mali2003 said, I would recommend TWO filter rules. The first will accept traffic explicitly from the one address in the 192.168.20.0/24 subnet to the entire 192.168.0.0/23 subnet, in the forward chain. The second rule would drop all traffic from the 192.168.20.0/24 to the 192.168.0.0/23 subnet in the forward chain.

Please also note that if either the source address or the destination address is assigned to the Mikrotik router itself, you would not use the forward chain, but rather the input chain if the destination is in Mtik address, or the output chain if the source is the Mtik address.