Maybe you could just try to write and see how you go.
I think writing about venting can be helpful.
There are no rules just what helps you.
If you google structured journaling there are a few ides there.
Some of the sentence beginnings I like is
If my feeling could talk it would say
what I would want to say back to this feeling is.
It also encourages at the beginning to write a date and time, I find that helpful and to write 3 words that would describe your feelings.
Of course you may not want a structure , but I find on some days I just do free writing, then on another day I may do some sentence beginnings and on other days I make Lists. I am a listmaker. Could be something simple like
Things I like to do
things that scare me
Things I find frustrating
These are just showing you how many ideas there are for writing in a journal.
Have a go, do what works for you and after a week see if you want to make changes.