Save the words you meet while reading
You are mid-chapter. A word stops you. You understand the sentence well enough to continue, so you do, and you tell yourself you will look it up tonight. By tonight the word is a blur, and the sentence that made it worth keeping has gone with it. A pencil mark in a paper book survives the evening. A promise you made to your memory does not.
That is the whole problem a personal dictionary is for. Not a stock of words in the abstract. One word, caught while it was still attached to a page, a person, or a reason.
What is worth writing down
Not every word you do not know. The ones that did something. A sentence you wanted to be able to say. A name for a feeling you did not have a word for. A word you almost used, and then replaced with a smaller one.
If you save everything, the notebook becomes another list you will not open. If you save the one that stopped you, you already have a reason to come back. Write it as you met it. Then add one line of your own: the page, the person, why it stopped you. A dictionary can supply a meaning. It cannot supply that line, and that line is what you will recognise later.
What has to be on the card
A saved word that is only a spelling is a reminder that you forgot something. You need a meaning you can read in one breath, the way the word is said, where it came from, and one example that shows it doing the work it did on the page.
Where a word came from is not a decoration. A word you can place, in a trade, a metaphor, or another language, has a reason to stay before any schedule asks you to repeat it. Vocabulary builds that card when you type the word: a definition, the pronunciation, the etymology, an example in a real sentence. The note is yours. The card stays on the iPhone, including when you have no signal, which is how you are actually reading.
Yours, not an assignment
Most vocabulary practice starts from a list someone else wrote, and you then spend the minutes on words you have never needed. The words worth keeping are the ones your reading already chose. Start there. When you want to practise, the session is built from that notebook, not from a deck that has nothing to do with the book in your hands.