Figure out what you are going to put all your food on. A table, a cutting board, will you need more than one, where will you place your grazing board, etc... See below for some ideas.
Choose the main focus of your breakfast platter. Our's was waffles and bacon. (See below for some other ideas)
What other brunch items do you want to include? Things like muffins, croissants, bagels, bacon, scones, and fruit. Items that compliment your main dish and are also easy to prepare or purchase.
Choose some other items that you want to include? Things like toppings, sauces, other condiments, and garnishes.
Figure out how you will arrange it on the platter. Do you need bowls? Small plates? Serving utensils?
Create your plan of attack for cooking the things you need to cook and buying the things you need to buy. Make a grocery list and also make a plan for making the food. Will you make it ahead of time or the day off and reheat?