New Areas
As a Grandmaster you have access to areas that other player levels can't enter. The Training Grounds, Dojo, the Meditation Room and the Pinnacle Chamber are the principal ones. Two other areas you may access are the Lagoon and the Bazaar, although the latter is accessible only after you have dropped off some money. Also, to get into the Lagoon, you will need the Lagoon map. It doesn't matter if you don't have it when you start the Grandmaster level, but keep looking for it. The most important first step is to craft a Mutated Venus Mousetrap.
Mutated Venus Mousetrap
You should already have a Venus Mousetrap and radioactive sludge. To make the mutated mousetrap, go to the crafting tab in your inventory and select the green arrow that smashes items up into smaller components. Now select the Venus Mousetrap in the inventory and click the "Prepare to Smash" button. Put your smashed-up mousetrap and the radioactive sludge in your crafting tab and press "Combine." Now you have the mutated mousetrap. Take it to the Training Grounds along with some explosives and brie cheese. Watch out for Assassin, Archer and Kung Fu mice because they have a ticket to the Bazaar, which you need to obtain.
Bazaar and Gnarled Tree
When you get the ticket to the Bazaar plus 95,000 gold pieces you can head straight there. If you have problems getting these items in the Training Grounds, head over to the Gnarled Tree and look for them there. Once you have the ticket and money go to the Bazaar and get the cartographer to repair your Furoma map. This repair will get you access to the Dojo, Meditation Room and Pinnacle Chamber. You should also go to the General Store and buy paint, glue and cheesy fluffs. You will need these later to craft cheese. You then need to go to the Gnarled Tree and stay there for a day. While you are waiting there, check on the cartographer's progress in repairing your map. Once he has finished, you'll need to return to the Bazaar to collect it.
Dojo
Now you can travel to the Dojo. Take the same mutated mousetrap, explosives and brie that you took to the Training Grounds. Your first goal in the Dojo is to catch students of the Cheese Claw, Cheese Fang and Cheese Belt mice. Craft some Maki cheese to attract students. You need to acquire either the blueprint for crafting an Ambush Trap from one of the mice, or you can craft Combat, Sucheese and Glutter chese so that you can catch a Dojo Master. Go to the Meditation Room and Pinnacle Chamber to get an Ambush blueprint from a Hapless mouse, if you don't have it already. Catch another Dojo Master and get an onyx stone to complete the Grandmaster's tasks. With this task completed you may progress to the "Legendary" level.