Sergio, I am assuming that you are refering to techniques for hand-pollinating cucurbits (squash, melons, cukes, etc.). The tape is placed around the tips of flowers which will open next morning, to hold the flowers closed.

Squash blossoms are easy to tape, because of their size. The smaller flowers of melons & cukes can be taped carefully, but I prefer to use a piece of paper towel around the entire flower, held on with a twist tie or a loose clothes pin.

You can cover the flowers with any material that allows heat & moisture to escape, and allows some light in. Floating row covers (such as Remay) are excellent for this purpose. Plastic bags could cause excessive heating in sunlight, and should not be used.

If your intent is to save seed from squash or melons, you can hand-pollinate as many as the vines will sustain - because you will eat them anyway, and just keep the seed. You could allow nature to set additional fruits, but only _after_ your hand-pollinated fruit are growing on the vine. You should remove any fruit that were set previously, otherwise your hand-pollinated blossoms may abort.

Cucumbers are normally picked immature, but to save seed, they must be allowed to ripen. This will cause a reduction in your crop, to varying degrees depending upon the variety, spacing, and soil fertility. If you only allow one to ripen for each plant, the effect may be minimal.

Be sure to mark any flower that you hand-pollinate, so that you save seed from only them. You can use yarn, twist ties, or anything that fits loosely enough around the stem to allow growth. For squash, the marker can be placed around the flower stem. For melons & cukes, with their smaller flowers, it is best to tie the marker around the vine at the flower's location.

Survey flags are very useful both to locate the flowers you have taped the next morning (when you will pollinate), and to mark the location of the female flowers to see if your pollination was successful.

There are several threads on GW detailing the procedures for hand-pollination of squash, particularly the one by MacMex (see link).

Other vegetables require different measures to ensure seed purity. I heartily recommend the book "Seed to Seed" by Suzanne Ashworth, which covers all common - and many uncommon - vegetables grown in the U.S.

Here is a link that might be useful: Squash hand-pollination