I find these apps very helpful...
Artist's Viewfinder, Mark II
For iOS, US$30/€33
As the first photo below shows, this is a digital Director's Viewfinder. Fairly new to the app, I have it on my iPhone and find it invaluable for scouting. If I like a particular view with a particular focal length, I can capture it from the app for later planning. It sure is a step up from a wire or cardboard frame and a piece of string. It looks like this app has been around for several years, and it's actively developed. No instructional videos on YouTube, but there's a thorough 75 page manual. For me, the performance easily justifies the price.
Artist's Viewfinder Screen with User Specified Frame Lines
Attachment 218396
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