I'm new to seed saving, but I've read many things on this forum as well as in books. Tomatoes don't cross pollinate very easily, so you may or may not need to do anything. I think the best way to be sure though, is to bag the blossoms that you want to save seed from. To pollinate these, all you should have to do is shake or tap the blossoms several times and they will pollinate themselves. Hope this helps.

With tomato flowers, it's the style length and anther cone aperture size that are important in potential crossing.

Being a naturally inbreeding and self-pollinating flower, the anthers shed pollen on the inside of the cone, that falls directly onto the stigma, if the aperture is large (think Brandywines and large Beefsteaks) then a solitary bee or other insect will be able to access the pollen. If however the anther cone aperture is tiny (Think Cherry Toms) then chances of a natural (insect) crossing is minimized or almost zero. That's why cherry tomatoes come up every year in the same location without any hybridization.

The next issues is style length. If the stigma protrudes from the anther cone then there is a high probability of crossing and vice versa.

Bagging a spray would be the easiest way to ensure purity, organza bags are exceptionally cheap and the easiest way bag blossoms.

Hope this helps.