The default access is using root/password (a really bad idea)
- Log into the DD-WRT administration panel
- Go to Services, Services. Find the Secure Shell (SSH) section
- Enable SSHd, SSH TCP Forwarding. Disable password login.
- Leave the port at 22, change the port externally via your router virtual server or port forwarding function.
- The DD-WRT team recommends port 443 because most firewalls leave that open - handy when at work or travelling.
- Add your keys with this format: ssh-rsa AAA...== yourcomment, each on separate lines (but no LF after the last)
You should be good to go - test internally first, then forward the port and try it outside your firewall.