Exactly the same way you'd make a regular one! That said, one common mistake people make with plant-based sandwiches is forgetting to add protein. This is so important because it will keep you full throughout the day and makes your sandwich much more satisfying. Some great vegan protein sources are tofu, beans, tempeh, seitan and mushrooms. (And if you're looking for even more inspo, the Vegan Society has a great list of easy sandwich and wrap ideas.)