Sorry, I have to go for a while but I'll give you the basic process.
Once you're saved one step before the auto walk to Lance, save and turn off your DS. Open up Radio Seed and enter all of the target info (Date, time, delay, etc. It's pretty straight forward). Then select the radio station you'll be using. It depends on the time of your seed, so to find out what radio station you're going to be using go to the first page of the Most Generation help thread and on the second post, OD posts the radio stations and how much they advance and what times they're on. Next, pick your mode. Do you want to verify your seed, or track the frame? First you should verify your seed, then after you verified your seed change to track frame.
Since you're aiming for Lati@s first, your target frame is 42 (The shiny frame) - 11 (Lati@s's starting frame). So once you're on frame 31, step forward and battle Lance and Latias should be shiny!