I always go out the afternoon beforehand and tape shut the flowers due to open the following morning. The main challenge with this is to recognize those flowers, as a prematurely taped flower will simply abort. The general rule I follow is that I can tape flowers after 2:30 PM up until dark.
Most varieties "yellow up" the day before. A few varieties hardly "yellow up" until the night before. But once I get to know a variety it isn't difficult to recognize the ones I need to tape shut. Notice in the above picture that there are both mature male and a female blossoms as well as some immature blossoms of both sexes. If you accidentally tape a flower not ready to open, just remove the tape in the morning. It will probably be okay.
Next I tape shut at least one male and one female blossom, due to open the next morning. If you can use more than one male flower that’s great.
I prefer to use masking tape, about 1 3/4 ". When I tape the flower, in preparation for hand pollination, I try to tape it shut a little closer to the tip of the flower, so I can remove it (and possibly a bit of the petals when I do) and still have plenty of material to work with when I reseal the flower after hand pollination. When I reseal it, if there is any doubt about the integrity of the flower (as in having any rips in it), I use extra masking tape to cover that part of the flower, always folding it around the flower (forming a cone shape with it) with care not to damage what remains. I never let the tape stick to the immature fruit as well as the flower, as I believe this might provoke an abortion of the fruit.
The following morning I usually go out at dawn. I believe I read somewhere that you can do this as late as 10:00 AM, but I always do my squash pollinations at first light.
I pick a male flower and pull off the tape, petals and all, leaving just a stem and the stamen (male part of the flower) covered with mature pollen. Notice in the above picture how I use a convenient squash leaf to set the stamen while I work on the female flower. (Notice how I can set down the prepared male flower, with petals removed, on a handy "leaf shelf."
After preparing the stamen, I unseal the female flower. Usually I do this by pulling the tape straight off the end of the flower. Then I use the stamen, attached to the stem, like a paint brush to coat the pistol, of the previously sealed female blossom.
After this I reseal the female blossom using fresh masking tape. Note: If you have a healthy population of bees, you may have to work quickly and stay alert to wave them off as they try to get into your blossoms. A bee can cause pollen contamination by getting into either the male or female blossom.
Here’s the finished hand pollination. It is important to remember that not all immature fruit will mature, not even under the best of conditions. So, it’s best to do a number of hand pollinations and not trust that one will do. You’ll know that a hand pollination took when you see the immature squash, on the end of the female flower, start growing.