A braindump is just what it sounds like. You are trying to empty your brain of all the things you are currently remembering to deal with.
The best way to do a braindump is by using a trigger list.
A trigger list is a list of questions/items that are designed to jog your memory of something you have to do.
Here is a trigger list you can use
You can also start by creating the main categories which represent the areas of your life, add subcategories to any large categories and then go through each category step by step, trying to remember what is on your plate there.
Tip
Marvin's Braindump feature gives you a box to type everything at once, one task per line, with prompts to jog your memory.