If you have not complete the Cooking Hobby collection for the first time, then complete it now. You will unlocked the chopping boards if you've completed it for the first time, but they are not free and you have to buy them from the kitchen section.
Okay, once you have completed the collection, you can restart the collection again, now the prizes for completing the Cooking collection will be 5LPs. After you completed the collection for the first time and start again, you no longer need a sim lvl 6 in cooking to get lvl 6 collectible items. In other words, you can complete the collection with a sim that is lvl 1 in cooking without using the sim that is lvl 6 in cooking.
First, you need 10 stoves (1 star), 2 or 3 sinks (1 star), a toaster (1 star). make sure all of the items is in the same room. Then you need 10 adult sims (if your sims are busy and you don't have 10 free adult sims, then any number above 5 are fine.)
Second, Make your sims cook Toast on the stove. Your sims only need 1 minute to do this.
Third, once your sims have finished cooking, an icon will appear on each Sims's head, showing that the food is ready to served. Click on these icon and your Sims will serve it. if you have an empty kitchen top or tables, they will serve it on these. If not, they will serve the foods on the floor.
Fourth, each time your Sims serve the food, you will get a collectible reward. If your sims find a new one, you will get Simoleons.
Note that after your sims served the food, you must clean it up. The quick way to make the foods disappear is make your sims clean up the plates, quickly cancel the action after they picked up the plates . With this way, your Sims doesn't have to go to the sinks to clean up these plates.
Keep cooking Toast, serving and cleaning it up until you find all the collectible rewards! Once you found it all, you get 5 LPs and you can start the collection again!
Further time you complete the collection, you get 5 LPs.
I got 25 LPs in an hour with this way.