The mega Dropdown is very easy to customize and works much like a CMS block.
1. Navigate to Category > Manage Categories
2. Choose the category you want to edit
3. Open the "Menu" tab
4. Make sure Mega drop-down is selected in the first option
5. Select the drop-down width. It's usually best to use a percentage value. This is the amount of total screen space the drop-down will take up. If you want it to span across the entire page, choose 100%. If it should be half of the screen, choose 50%, etc.
6. Choose the basic layout of your left, subcategory, and right sections. Your selections must add up to 12. So, if you want the first part of the dropdowns to be the subcategories, and the right, wider part to be the sub category, choose
7. Since we just designated our right block to be the right part of the drop-down, we simply need to scroll down and edit the right block. Use this block just like a CMS block. by default you can write HTML, or click the WYSIWYG. (HTML is recommended, but use the WYSIWYG editor to easily upload images)
8. Save the category