Thinning Fruit Trees

I read that I am supposed to thin my dwarf peach trees (first year with them), but I’m confused about when to do it. Research turned up late May to early June for where I am and said “when the fruitlings” are about the size of a marble or a nickel. It’s early May and they are that size. They are covered in fruit! I am so excited and also so sad I have to toss a bunch, womp! And I just pick them off, is that right? Needed to verify this with some actual humans. Also, thinking of placing mesh bags over the entire trees. When is the right time, exactly? Thanks for any advice!