Shrub Pruning Is Complicated
There seems to be a conspiracy about pruning shrubs. Most sources either pretend that if you can avoid it entirely or it's easy with a few basic principles.
You can avoid it, but over time your plants will dictate your design. Some shrubs grow faster than others, and they will dominate the unpruned landscape. They will outgrow their boundaries, crowd out other plants, change the proportions, and make themselves the stars of your production. Sometimes these overgrown landscapes look fine, but most of the time they go from being Audrey Hepburn to Audrey II in Little Shop of Horrors.
There is a notion that you can buy the right plant and it won't outgrow its space, but it really doesn't work that way. You can buy a mature tree or shrub and it will have immediate impact, and maybe it won't grow much during the first year or two (because of stress), and maybe it's a slow grower, so it won't look out of proportion for a few more years, but eventually it will grow too big or die. I don't want to throw too much shade on garden designers, but they generally don't care as much about what happens in five years than they care about giving you a beautiful garden now.
I think that the conspirators are trying to hide two simple truths: pruning is a lot of work and no one agrees on what the end result should be.
I will delve into the end result problem in a different post because it is so contentious. An ilex clipped into a sphere is classic geometric perfection to one gardener and a horrific contortion to another. If you do a search for images of "badly pruned shrubs" you will probably see something there that you rather like. If you look for a video about pruning shrubs, you will see someone talk about proper cuts and form, and then they will cut two minuscule branches off of a four foot high shrub - I think it's because they are afraid to show the "after."
The other truth about pruning is that it's a lot of work with a tremendous amount of debris. If you have a shrub that is a three foot sphere and you want it to be a 30" sphere, you have to remove almost half of it (42% of it - let me know if you want to see the math), and that is a lot of twigs and leaves. The softer new growth of the hedge that you want sheered will have less volume, but sheering needs to be done at least once a year (usually more), and sheering off only the easy stuff is why most hedges are starting to eat the driveway. Also, once you start, you really have to finish because half pruned shrubs look weird.
Another job is to figure out what kind of shrub it is and the best way to prune it. Usually shrubs should be pruned before they put out new growth, and usually they put out new growth in spring or early summer, but some shrubs grow throughout the summer, pruning can affect flowering, different shrubs react differently to pruning and sheering, and some are very fussy about when they are pruned.
I think the easiest way to prune with the least potential for regret is to thin a shrub and reduce its size a bit. For a beginning pruner I recommend that you choose one shrub, do your homework, wait for the proper time, and get good tools (felcos, a pruning saw, and maybe some loppers) and a tarp. Don't start until you know what you are going to do with the debris; otherwise it will sit on the tarp and mock you until you figure out how to get rid of it.