Those two parents will do great, actually. First thing to do is open up RNG Reporter and click on Time Finder. Click on the Shiny Egg tab and check the date. It should be today's date, but if it isn't you can either change it or just use the date displayed there. For nature, select Quiet. Ability is for Pokémon with 2 abilities. If you're ever unsure about which ability is which, the Dex/IV Check option on RNG Reporter will list them. If you care about the gender, pick one. The gender ratio for Dratini is 50-50, so you can leave that alone. Since we don't care about Dratini being shiny, enter random numbers in the ID and SID box. Make sure the option that says HGSS Shiny Egg is ticked. Now, click on Generate, and it will make a list of dates, times, seeds, delays, and frames for your egg. On my computer, RNG Reporter freezes for a minute when doing this, so it might do the same to you. You want to pick an egg with the lowest frame possible, preferably 1.