This is the same for any hobby, firstly you need to complete the collection as you will not be able to start the collection again if you don’t have all the pieces yet.
Once you have found them all it will tell you, then you will need to go back to the blue collection screen (where you can find all the hobby collections)
You do this by clicking on the sim info panel, find the hobby tab (second one) and click on the hobby picture (on a sim who is doing this hobby)
Once opened you will see the start collection again button at the top, click that and the collection will be reset so you can find all the pieces again.
When you reset the hobby it doesn’t reset your level, so if you were on level 6 you will still be on level 6.
Confused about hobbies? Read my Hobbies Frequently Asked Questions post
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The Girl Who Games 