You probably didn't find much information because hybridizing, especially with plants like hydrangeas, is not an easily undertaken process :-) Some plant species are very promiscuous and will cross pollinate readily, generating all manner of interesting offspring. Others, particularly woody plants, are less inclined to such frisky behavior and new cultivars are typically created from sports or highly controlled breeding programs (like in a lab).

Hybridizing aside, growing hydrangeas from seed is relatively easy. Even those that are known for producing mostly sterile flowers will have fertile flowers present - they are tiny compared to the larger and more showy sterile florets. Look at a flowering lacecap to recognize - the interior portion is made up the tiny fertile flowers and they look exactly the same on mops and annabelles.

Find that Dirr book on hydrangeas (Hydrangeas for American Gardens) at your local library or order from Amazon. He goes into a lot of detail on all forms of propagation, including growing from seed. And it takes at least a couple of years for a seed-grown hydrangea to achieve flowering maturity.

And I don't even want to broach the subject of attempting to hybridize patented cultivars :-) I don't know for sure but I'd be willing to bet it is not authorized. Although if you were doing it only for your own interest and not for any potential commercial purpose, I'm not sure who would ever know.

Blushing Bride is Endless Summer x Veitchii. This would be incest!!! Just kidding...who knows what you will get. Probably just an Endless Summer.

I agree with Tim's assertions about breeding process and especially in part that it's indeed a fun hobby for amateurs :-)
As an example I could tell you about my efforts to breed Blue Bird with Tokyo Delight and Blue Bird with Lady in Red.
The goal was to create lacecap as hardy as BB (fully hardy in 5b-6a without winter protection) and as showy as TD (fall flowers) or LiR (fall flowers and foliage) because BB after initial bloom didn't present any additional interest the rest of the season.
Leaving all technicalities alone I'm going straight to the end results: crosses of BB x TD, TD x BB and LiR x BB resulted in plants that were very similar in all aspects to BB and were discarded.
However, out of 36 seedling of BB x LiR 4 plants from the very beginning start showing red petioles as those in LiR and were left for the further evaluation. They didn't bloom for the first two years. Second year's fall foliage of those 4 seedlings was different than BB parent, though not as prominent as in LiR. After two years cuttings from those 4 plants were taken, rooted and given to several friends in much colder (5a-5b) locations than mine.
Some of them were overwintered in garages (5a), some went straight in ground (5b). All 'garaged' plants survived, most of 'inground' plants too. All of them bloomed the third year with blue flowers similar to BB (even those that were in pots with a neutral pH potting medium) while fall color of flowers and foliage was similar to LiR.
Now I'm anxiously waiting for reports about their well being after this brutal winter.
Just in case, I'm reserving the name 'Blue Lady' for this plant ;-))

So how is Blue Lady????

Where can I get some detail instructions on how to cross pollinate hydrangeas?

Probably on books, many .gov and .edu websites. Articles on viburnums might also be helpful with hydrangeas.

https://naldc.nal.usda.gov/download/20503/PDF

Michael Dirr is the preeminent US hydrangea authority and hybridizer and his book, Hydrangeas for American Gardens, discusses propagation methods in some detail, including growing from seed, and also addresses cross pollination and hybridizing.