https://wiki.mikrotik.com/wiki/Manual:IP/Firewall/NAT
In a nutshell, you want to use the IP > Firewall > NAT configuration. Add rules to the "dstnat" chain. In each rule, you use dst-port=xxxx as the "outside" port you want mapped, action=dst-nat and to-address=x.x.x.x (inside IP address) - if the inside port is the same as the outside port, you don't need to specify a to-ports. If the inside port is different (e.g. mapping outside 8080 to inside 80) then specify the inside port as the "to-ports" value.
The NAT table is evaluated once for each new connection when connection tracking is in use (the default condition).
Rules are processed in order top to bottom, and the first match is used.
If no rules match, then the packet is passed w/o any modification.
Rules match only if 100% of the conditions are true.
If any condition has multiple values, then this is an "or" condition - e.g. dst-port=80,443 will match if the destination port is 80 or if it is 443.