That may be different in the USA, but in the EU it's surprisingly straightforward and easy (done that once 10-12 years ago, no lawyer involved).

It is easy here as well, the forms can be completed online quite quickly.

But from what I have read about it, the process is surprisingly error prone.

Probably the most common lay-person mistake I've read about is not submitting an actual specimen of the trademark in use, instead of an actual tag or label they submit something like an image of the mark alone, or a model or mock-up of what it is going to look like. All of those are invalid. Yet despite big warnings about submitting the correct things, with links and examples of what is acceptable and what is not, many people still file with an image of the wrong thing.

Another of the biggest mistakes is already part of the discussion, and that is filing for trademark protection too soon. Federal trademark registration must happen AFTER the mark is used in interstate commerce. As mentioned above, you can file an 'intent to use' form to effectively pre-register the mark, but you cannot register until after actual use in interstate commerce.

One of the costly problems is not doing a careful and complete search. As a famous example from a few years ago, Payless Shoes registered a trademark of four stripes on a shoe, but Adidas already had an identical trademark with three stripes. The search of four stripes found nothing, the mark was granted. Very soon there was a lawsuit that ended very badly for Payless: $30M in "actual damages", $137M in profit reductions (basically saying the money from the sales should belong to the other company that owned the mark), plus $137 in punitive damages (direct punishment). The two companies eventually reached a private agreement, probably for much more than the $30M in actual damages but less than the $300M in the initial verdict.

While it is something an individual can do on their own, some of the steps should be done by experts who know what types of similarities to hunt for.

Like many legal issues, the worst case is not that your product fails, it is that your product succeeds and then is discovered to be seriously infringing on another IP.