What kind of stuff you wanna buy? You may want to start in Prince Edward, where Fa Yuen St is, and then:
Fa Yuen St --> Lady St / Area around where you can find lots of electronic stores --> Temple St (but it's only "active" in the evenings) --> TST
As you know Fa Yuen St / Lady St offer mainly cheaper products, you may want to have a 2nd thought if you wanna go shopping there. Hope this can help you!
Cheers, Blom
