Run fresh water over the seeds to get rid of the rest of the pulp. You should end up with this.
Now dump the seeds onto a folder paper towel or cloth. Separate the seeds so they are not sticking together and allow them to dry several days. Once they are dry you can put them in a container or plastic baggie to keep them safe until next planting season.
Be sure to label your seeds along the way. They all start to look alike. You may rather have 10 slicing tomato plants instead of 10 grape tomato plants. I had close to a 100% germination rate with the seeds that I saved from last year's tomato plants.