• Do it in winter or the time of year when the least amount of topgrowth is occurring.
• When I root prune, I take off an inch or close to it all the way around the rootball and a bit more off the bottom.
• Always take the root pruning or repotting opportunity to replace the potting soil. It doesn't last forever and needs to be replaced or refreshed periodically. Any kind of fast draining, textural potting soil will work. Avoid those that are peat based or have fertilizers incorporated.
• Washing off the roots is optional. If a brand new, fresh from the nursery plant, by all means!! You want to get rid of the current potting soil, which may or may not be of good quality, and to loosen up the rootball. Whether or not you need to do that for an established plant is up to you and the state of your plant :-)

Very helpful thanks! What do you think of a 511-type soil for rosemary? I have some left over from

repotting citrus. Or should I use something similar to what it’s in right now? (A general-purpose potting soil I think.)

Yes...the 5-1-1 is great!!

Host lift you plant out of the pot. If it looks room bound then cut away. I've root pruned anytime of year with no problems.