Assuming you are adding your food scraps every couple of days as you lift the moisture mat you will see the worms working just below the surface. (If you have to dig down to find worms you need to ease up on the food addition as you don’t want waste building up without the worms having time to process it.) As the waste gets consumed by the worms the volume reduces and it’s processed into black gold (worm casts) and liquid (which you will use as liquid feed). This means as you fill each tray it sinks at quite a rate so the time to add a new tray on top is when the tray the worms are working on is just about full and when the bottom of the next tray will sit on the compost below. At this point you take off the moisture mat, add the next tray and some waste and replace the moisture mat which always sits on top of the freshest waste.
Don’t worry if the worms don’t appear to move up into the next tray for a few weeks – as long as there are no gaps between the trays and you can see flecks of compost on and around the top tray all will be well. After all worms do not know about trays so as long as they are working on the compost just below the surface all will be well. Just occasionally it’s worth checking that no gaps have occurred between the trays where the compost has sunk as this will mean the worms have more of a challenge to get up and down between trays. If there is a gap – add a bit of waste into it to fill it in.