Once the rice is ready, put it in a serving platter and top with sesame seeds. Scatter a few tablespoons of tobiko, then sprinkle on the shitake mushrooms and some of the shiso. Crumble the tamago and uni on top. Finish with a few tablespoons of ikura and crab (optional) then top with the remaining shiso.
You'll want to serve this as soon as possible. If you have leftovers, put them in the fridge and when you want to eat it, let it come to room temperature before eating. You won't want to eat it cold as the rice will be hard and you certainly don't want to heat it in a microwave.
If you're confused, here's a video that might help.