Sure~
First, you need to have a shiny Pokemon that you've caught yourself--one that you encountered randomly, or a few shiny Pokemon that you chained using the Pokeradar (can only be done in Pearl, Diamond, or Platinum.) In HeartGold/SoulSilver, if you haven't captured the Red Gyarados yet, you can use that as well (but don't save after you catch it).
From this, you need to determine the Pokemon's IVs (individual values). Using this and your trainer ID number, you can figure out your SID (or Secret ID) which you will need in order to RNG shinies. If the shiny Pokemon you caught has battled any other Pokemon, you need to keep track of its EVs (Effort Values), when you calculate the IVs.
But once you have your ID and your SID, you can start RNGing. The process is a little different in D/P/Pt and HG/SS, but there are some basics that are very similar.
Please feel free to email me about it too(I've got it listed under my Contact Info).