It would be the exact same way as in Emerald. The only differences come from RNG reporter due to the different seeding process.
Actually I'm slightly wrong. R/S games seed the RNG on the 6th frame after SRing. But unless you regularly do both R/S and Emerald RNG abuse it doesn't really matter.
Basically you SR when the first timer runs out (personally I just set the first timer to 0 and SR and start the timer at the same time). For the frames you enter the target frame you want (example if you want the spread from frame 100000 you enter 100000 into there). Leave the lag spot blank for now, check GBA box if you're using a GBA/SP. Do whatever action is necessary to attempt to encounter the target at frame 100000, check the frame you actually hit, enter that in the bottom input area, hit update (this changes the lag area), then repeat.
Next time you'll probably get a better response if you ask here