Kabouter Games
Explorer
Answer: They ask the DM how to go about it on a case by case basis & then do that.
There is no set way.
Yes there is. I gave you a page reference above.
Step1: Think up item.
Step2: Name item. (doesn't have to be step 2)
Step3: Write down the details. (optional)
Step 4: Introduce it into play.
That's always been my approach.

Seriously, I'm the DM. I am not constrained to making magic items via some formula. My will/whim is sufficient. Nor am I required to provide the players some formula so that they can replicate what I've made.
Hence the DMG rules on magic item creation by PCs being optional.

Yes, I am aware that this might not sound very useful to you as a novice DM. But the fact that the DM is all powerful & that the rules answer and bend to them, not vise-versa is something you need to learn, accept, & practice responsibly.
SAY IT AGAIN FOR THE PEOPLE AT THE BACK
The wand of Sphinx detection
...is serious awesome.

Cheers,
Bob
www.r-p-davis.com