Gathering is the hardest. Sometimes it's just too difficult to clip off those lovely blooms. Still, that's the point isn't it? So, first taking from the least viewed point of the whole plant, clip away, leaving lots of stem.
Chose various blooms. Like flowers together make a powerful statement, but random colors bespeak their own glory. Depends upon the mood.
A handful of long grasses wrapped around the fingers, then tucked into the base of a glass vase, before filling with warmish water covers the stems and adds a professional look.
Hosta leaves make good frames for the base of the arrangement. In the winter, boxwood or yew branches add greenery.