Stuff he said. But I deleted for a smaller post.
Well, HGSS and platinum are a little different in how they go about RNG'ing standing legends, but I'll go through both.
Starting off with HGSS. To start as an example. Say we are using this spread:
PID: 31983db5 (ADAMANT, ability 1) 31/31/31/11/30/31 (Seed: 930f09bc Method 1: 165, J (synch): 136, J (no synch): 146, K (synch): 136, K (no synch): 138) HP STEEL 70
So my desired nature is Adamant, so I can going to either catch or hatch a Pokemon that has the ability Synchronize along with the nature of Adamant. What this does is basically just helps the wild Poke you're trying to catch have better odds of being the nature of that Synch. When using a synch it always has to be the first in your party.
The synchs are meant to be used on both DPPt and HGSS catches.
So now, I am going to go where my standing legendary is, since most of the legends aren't anywhere near moving trainers or other people. Or of wild Pokes you're not going to need to worry about them messing up your frame.
So now, for this example I am going use the stationary Legends Ho-oh, and Giratina. Just for examples.
So for Ho-oh, you're going to run up the tower and it's long stairways. Once you get to Ho-oh, you can either save right in front of him, or you can save right next to the stars leading up to him. Since we are going to use this part of the seed/spread I posted earlier: K (synch): 136. That means that we have a synch in our party, and our frame is that of 136.
There are few ways to advance your frame. That is to call Elm, which is also to see what seed you hit, whether it be the right one or not. Elm is one frame advancement per call. Joey calls, one call to him is 2 frame advancements. You can also do the Pokemon Talk radio station in your radio part of the Pokegear. This advances by 13-14 frames. It's not consistent in being 13, 14, 13, and than 14. It's basically random. So you'll have to compensate for that. It seems to be the quickest way.
But since the radio can be a little difficult to use right off the bat, I'll just ignore that part for now, and we'll stick to our Elm calls for advancement.
So we saved in front of Ho-oh. We reset for our seed which is Seed: 930f09bc. Once we load into the game, immidiately open your Pokegear, and call Elm about 10 times. See if you hit the seed or not, if you didn't which one did you hit?
Once you land on the seed you are wanting. If you had called Elm 10 times already, now you just need to call him 125 more times.
125 + 10 = 135. Since Ho-oh's starting frame is one, after calling for the 135th, you will get your desired IV's upon him. (means when you talk to Ho-oh and encounter him, he will advance the frame by 1)
Now for Platinum. Platinum is a bit different but much thus the same.
So we are going to save next to Giratina in the Turnback Cave with our synch first in the party.
We check our seed via Coin Flip. Until we get it.
Than we figure out how many journal flips we need to do that have Pokemon Caught, killed on those pages. That kind of stuff.
Our frame is J (synch): 136.
So Journal flips advance the frame by 2, and Giratina's frame is 1. So we are going to do need to do 67 Journal flips, and than walk 128 steps with only 1 Pokemon in your party (128 steps advances the frame by how many Pokemon are in your party if there isn't any wild Pokemon in the area).
So if you had only 1 Pokemon in your party, you walk 128 steps, and you Journal flips 67 times, and than talk to Giratina. He will have your desired IV's.
Same goes for pretty much every other standing legend as well.
Now with Wondercards, they are a bit different. Say we are going for the same seed as we have for these 2. Except we would use Method 1, and sync's are of no use for these guys. We advance the frame like we normally would. If it has a fixed nature (meaning that it always will be one nature) that it'll come out with those IV's and the fixed nature. Now if it doesn't have a fixed nature. It has a 1/26 (26 natures right? Or however many natures are in total) chance of being the nature you would want. So you might have to hit the same seed, advance the frame a bunch of time to get the desired nature for your non-fixed nature wondercard Poke.
Method 1: 165
Wondercard Pokes have a starting frame of 1. So you'd advance 164 times, than pick the Poke up.
Make sure you're in a mart where none of the people in the Mart are moving. As if they are moving, it'll mess you up.
I hope this helps, and I'm sorry if I forgot anything.
Good luck<3