If the toilet is conventional, the gasket is just sitting between the two. 1. Turn off the water supply to the toilet.
2. Flush the toilet to get as much water out of the tank.
3. With some old rags, sop up all the water left in the tank.
4. Loosen and remove two nuts and washers on the two bolts which secure the tank to the bowl.
5. Carefully lift the tank off off the bowl, and set it upside down on the floor.
6. Remove the old gasket from wherever it's stuck, and clean all fragment from the contact areas on both the bowl and bottom of the tank.
7. Install the new gasket on the bowl flat.
8. Remove the bolts and old washers from the tank, and clean the mating surface.
9. Install new washers on the bolts[they go inside the tank, under the bolt heads]]
10. Carefully set the tank atop the bowl, being very careful to get perfect alignment on the first try.
11. Reinstall the two bolts with new washers.
12. By hand very carefully reinstall the nuts and washers on the bolts, running them all the way up by hand.
13. With a wrench, very carefully final tighten the nuts, but not too tight as it is very easy to overload the china bowl or tank, resulting in breaking one or both, ruining the entire toilet assembly. The nuts don't have to be extremely tight just up snug so as to just slightly begin to compress the washers.