If the seeds are not floating in liquid from the tomato, add up to a cup of water to help separate the seeds from the pulp. Then set the bowl of tomato seeds and pulp in a warm, out of the way spot. You will need to allow 2-4 days for the fermentation to take place. As it does so, the mixture is going to begin to smell awful, so store the bowl where you won't pass by frequently.
If you have glass jars available, they make a good container for fermenting tomato seeds. The extra space at the top of the jar controls some of the odor and the clear sides let you keep tabs on what is happening. Covering the top of the jar with cheesecloth or paper towel will keep fruit flies out and also diminish the spread of the unpleasant odor.