Ok so lets get started.
First head to http://www.dd-wrt.com/site/support/router-database/ and in the search put "TL-WR703N" in the search dialog and hit enter.
You should see one result found click on this and the page will expand download the "factory-to-ddwrt.bin" firmware.
Once the firmware has downloaded, plug in your TL-WR703N and connect the TP-LINK_XXXXXX wireless network.
open your web browser and goto http://192.168.1.1 if your asked for a Username or Password just use admin for both.
The next bit can be a bit tricky as the interface is all in Chinese so we have highlighted where to click in red.
Click on links 1, 2 then 3, then select the "factory-to-ddwrt.bin" image you downloaded earlier, then click on link 4
Click ok to the message prompt that pops up.
Once the firmware has been uploaded the TL-WR703N will restart and you will have to connect to the new "DD-WRT" wireless network. Once connected goto http://192.168.1.1
Ok now we are ready to move onto step Setting Up DD-WRT