Reading the patch notes would have helped, but a decently short explanation is:
1) Treat the Rain Catcher as a Dew Collector, but you place Iron Buckets into it to fill.
2) Dig some trenches in your garden to hold some water.
3) Take out the filled bucket from the rain catcher and poor out the water into the trenches.
4) Crops in A21 DFalls don't destroy water as they grow (it's intended, but the feature was broken changing A20 to A21), but they still need water. If you're just growing crops then you're now done watering them.
You can fill empty jars or buckets (including Clay Bucket) from sources of open water as normal, so you could fill them from your crops-trenches. You could have ignored step 1 entirely if you'd filled a Clay or Iron Bucket from any open water instead, that's what I did earlier today.
If that's not working, then try updating without using the Mod Launcher (that's what you meant right?), just do a manual install (there's a video in the first post or two showing how)