I have often taken a screenshot of the area of discussion, then used the markup feature on my iPhone/iPad, to make quick notes and changes. Also, the "old-fashioned" way of using a draft set of plans (or a photo copy of the area), marking changes in pencil, then taking a photo of the new markup for email, is hard to beat!

Tracing paper. So easy. Snap pic. Email.

Try verbal or written communications. Many times explaining 'what/why' you want something done/changed speaks more that 'how' you want something done/changed. That way the architect is better able to utilize their creativity and problem solving skills. This method may not work for all changes, but it may be a better way of getting what you need.

As Doc said, just use a pencil and tracing paper (bumwad in architect speak).

A coupla photos, close up of what's currently on "my board" and one of a recent design session with a client. No computer, just overlay after overlay after overlay....fast, easy cheap and even the client can work the pen as we all learned that skill back in the 1st grade!