If you are using a tray wormery and are the worms are happily processing the amount of food you are putting in, then the bottom tray should be ready to go when the top one is really close to full - usually not before.
The best method is to dump the entire tray into your compost heap or border - but if you want to retain every single worm, take the lid off and put the bottom tray on top of the top tray and leave open to the air.
Gently remove the top layer of compost until you can see the worms and they will bury deeper as they move away from the light. Repeat until there is no compost in the tray and the worms will have moved down to the tray below. Some worms may go into your garden with the compost this is not a problem - red worms, tiger worms, dendras and composting worms are one of the 27 species of UK native Earthworms.
For more help with Worm Composting please visit our Wiggly Wizard knowledge base here https://wiggly-wigglers-faq.malcolm.app/
Or if you want more info about purchasing a wormery or starting worm composting please pop on over to https://www.wigglywigglers.co.uk/collections/worm-composting