This is probably the easiest step. Once you have DD-WRT set up and the computer configured you can log in to the DD-WRT interface and send the magic packet to wake up your computer.
1. Log in to your router's administration web page through its WAN address. If you used the settings I showed you earlier you this will be: https://YOUR.WAN.IP.HERE:8080
Obviously you need to use your actual WAN IP unless you register the domain YOUR.WAN.IP.HERE and set up DDNS on the router!
2. Click the "Administration" tab.
3. Click the "WOL" tab.
4. A list of hosts that have connected to the router over the LAN will be displayed under "Available Hosts". Check the box next to the host you want to wake up.
5. The host will then show up under "WOL Addresses". Click the "Wake Up" button next to the host you want to wake up.
The router will send a magic packet for that host waking it up from whatever state it is in. After a few minutes your computer will have booted and be available for a remote connection.