OK, this will be a long one, so my apologies in advance. ;-)

There are really several separate issues regarding seed saving:
(1) Whether or not the variety is open pollinated. Seed from hybrids can be saved, but the results will be unpredictable in the next generation, and generally unsatisfactory.
(2) Isolation, no prevent unintended crossing.
(3) Processing, including curing, cleaning, and drying.
(4) Proper storage.

Much of this information is available in the Seed Saving Forum, so I won't try to cover it all here - a little time spent searching there will provide much of the info you seek.

Most sweet corn varieties sold commercially are hybrids, and can't be saved - only open-pollinated types will breed true. Corn is wind-pollinated, and easily contaminated by commercial crops grown in nearby fields, or by other gardeners upwind. Bagging (both ear & tassel) is generally required to ensure pure seed, although isolation by time is possible, utilizing transplants.

Corn also requires a large minimum population to avoid inbreeding depression (200-300 plants). The ears should be allowed to fully dry on the stalks; but if poor weather or animal attacks threaten the crop, the ears can be removed & dried in a well-ventilated area. As you can tell, saving corn seed is a considerable amount of work; but the yield is very high, and if the resulting seed is frozen for long-term storage, it will last for many years. You may only do it a few times in your lifetime!

Cucumbers are also very prone to crossing (by bees) if other varieties are grown nearby. Blossom-bagging & hand pollination could be required; there are several good threads on GW that cover this. To save seed, at least one cuke per plant must be allowed to ripen completely - it will get much larger than the normal size, and will change color as it ripens. Allowing one fruit to ripen will reduce the yield to some degree, usually minimal. For the best seed vigor, the ripe cuke should be cured for several weeks after picking.

To process the seed, split the cuke lengthwise, and scoop out the seed. This should be done over a bowl; you want to catch the juice. Press out as much additional juice as you can, scraping on the seed cavity with the back of the spoon. You want the juice to cover the seed. If necessary, add only enough additional water to cover the seed, and stir well.

Fermentation will naturally remove the membranes which surround the seed. Allow the covered seed + juice to ferment for at least 24 hours. Then pour the seeds into a deep container, add water to nearly fill, and beat the mixture with a wire wisk to loosen any clinging material. Pulp, sacs, and bad seed will float to the surface, while the good seed (which is heavier than water) will sink to the bottom. Gently pour off most of the water & debris, add more water & stir; repeat until the water is clear, and only the good seed remains. Pour the seed through a wire strainer, set the strainer on a towel to wick out excess water, and spread the seeds in a single layer on a non-porous surface to dry. A fan for the first 24 hours is recommended.

Tomatoes are _usually_ less likely to cross-pollinate, as long as they were not grown directly adjacent... search GW for "tomato crossing" for additional info on the exceptions, which I have listed in several threads. Choose unblemished tomatoes (especially on the blossom end) for seed saving purposes.

Once the tomatoes are fully ripe, the seed can be processed. There are several methods that can be used, the most common of which is fermentation similar to the method described above for cucumbers. Cut the tomatoes across (not lengthwise) to expose the seed cavities. Wearing rubber gloves, push your finger into the cavities to force out the seeds; smaller tomatoes may simply be squeezed. Again, try to get as much juice with the seeds as possible.

The fermentation time for tomatoes is longer, generally 2-3 days. This is strongly influenced by temperature, and the seeds require observation to prevent over-fermentation & sprouting. There is considerable disagreement over "to stir or not to stir"; I recommend stirring daily, to ensure that all seeds are submerged. Once examination reveals most gel sacs to be broken, pour the mixture into a tall container & process in the same way as the cucumber seeds - the good seeds will sink.

Peas (I assume you mean Southern peas) have few problems with crossing... but if grown for seed saving, different varieties should not be grown adjacent to one another. The pods should be allowed to dry on the plant, and picked frequently to avoid splitting.

Okra is self-pollinating, but bees love the blossoms, and will cross different varieties grown nearby. Generally, grow only one variety if grown for seed, or use caging or blossom-bagging to ensure seed purity. The pods should be allowed to dry on the plant until they begin to open, at which point they should be removed & placed in a well-ventilated area to finish drying.

What about spaghetti squash seeds?

Spaghetti Squash are in the c. pepo family and can cross with most summer squash and many winter squash varieties, such as Jack O Lantern pumpkins, acorn squash, etc. These squash can be crossed by bees for up to a quarter mile. So hand pollination or 1/4 mile isolation is required. Below is a thread on hand pollination. It's not hard to do, and you still get to eat the squash!

George
Tahlequah, OK

Here is a link that might be useful: Hand pollination of Squash