The rule: the refrain may not change, the context must
Write a line of five to eight words that states a plain fact. It must be grammatically closed and contain no words like still, yet, or again — those belong to the context, not the refrain. Then write two lines before it and two lines after it, twice.
The refrain in the worked example, the toolbox is still in the hall, is a flat statement of location on its first appearance. By the third it is a confession: in between, the speaker has measured twice, bought the parts, and finally put the tool away. Nothing in the refrain changed. Everything around it did.
Where to put the appearances
Line 1, line 5, line 9 is the simplest pattern and works in nine lines. If you want the repetition to feel more like a return and less like a metronome, use line 1, line 6, line 9 — the middle appearance late, so the gap between two and three is short and the last one lands quickly.
Never put the refrain in the last line and also in the second-to-last. Two appearances back to back with no new material between them tells the reader you have run out of poem.
The rhyme budget
Two rhyme sounds across nine lines is plenty. Give the refrain its own sound and let the other four pairs share a second sound. Notice in the worked example that the rhyme is carried by hall / spring / chuck on the refrain-side and by did / lid, plugs / ... on the other — and that the second sound is deliberately thinner, because a heavy rhyme on the non-refrain lines pulls attention away from the return.
Then make it nineteen lines
Once the nine-line version works, extend to a villanelle by adding two more tercets and a quatrain, and by letting a second refrain appear at line 3. That is the whole distance between this drill and the fixed form: the drill teaches you the one skill the form actually requires, which is making a repeated sentence collect meaning as it goes.