Make a shaker. Wood frame with 1/8" mesh will remove the castings from the bedding.
Use the castings and return the bedding and the worms to "get back to work".

You can certainly harvest if you need the gratification NOW.

If the bin is in the right temperature range, and they are eating what you give them, the bedding will be turned into vermicompost (POOP) in time.

A 3X2X1 bin "filled" with bedding is a lot of fiber. Just be patient.

The first batch does seem to take extra long. Be patient is right. Remember that not only did your worms have to take some time to get used to their new home but all the micro flora and fauna had to become established before the worms could really get going. And cooler temps over winter will slow things down a lot.

If you are really really keen on using some castings to give plants a boost this spring, you will have to use sparingly (even if a bin that size had been well digested, you will probably only get a bucket or two of castings) but you can do it. A little worm bedding as mulch in the garden or on your potted plants won't hurt anything.

The other option is to wait and harvest later in summer and give some plants a boost then. Or harvest a little at a time and use a cup of the castings in the hole when transplanting.