There is no reason not to safe your own tomato seeds. Tomato seeds are very durable and even under less than optimal conditions your seed saving process will result in viable seeds.
Collect fruit from a plant that did really well. If you have multiple plants of the same variety make sure you get that one that gave you the biggest, tastiest harvest that year. We do this in order to maximize that chance we are retaining the best qualities we can have in the seed.
Cut open your tomatoes of a single variety. And remove the seed pulp. Note! Keep your cutting surface clean and clean it after each variety. You cannot see from the outside which variety the seed belongs to. So be careful and meticulous.
Put your seed pulp into a jar. Add some water (quarter-half the original amount). Add a lid if you want to, but don't close it tight. You don't want to process to go anearobic.
Wait 3-7 days. We need to get rid of the gel around the seed. It prevents the seed from germinating. The gel will start rotting slowly. Check every day how it looks and swirl the liquid.
Once the gel is mostly broken up, rinse the content of the jar a few times, carefully pooring away the liquid above the seeds. The seeds will sink. So add water, swirl, wait, poor liquid away. Repeat this a few times till the liquid is clear. Your seeds are now ready for the next step.
Poor away most of the liquid. Then poor the rest, containing the seeds, on a paper or regular plate. Spread out your seeds so that they don't make contact (use a toothpick or something similar). Try to get rid of any liquid if there is a lot left. Otherwise, mark the plate, put it in dry warm place. The seeds will have dried within 1-3 days depending on the conditions.
Collect the seeds from the plate after they have dried and put them into their final place: ziplock bag, envelope, small jar, etc. Label your container: variety name, year, notes.
collect your best tomatoes of the same variety.
Cut open your tomatoes in halves or quarts.
The seedpulp removed from your tomatoes.
Put the seed pulp in a clean jar.
A bit of water added to the seed pulp.
Label your jar and put it in a warm place for 3-7 days checking it regularly.
After 3-7 days, rince with water a few times, stirring the content till you end up with clear looking liquid. Seeds that are ready will sink.
Poor away as much liquid as you can, you may lose some seeds, especially those that tend to float more because they still have a lot of jelly coat remaining. That is fine. Poor the content on a clean plate.
Spread out your seeds.
Do not forget to label your plate!