Let me start by telling what a viewfinder is. A viewfinder is a see through frame which will assist you to find the most beautiful area to paint. Now in order to construct a viewfinder read on.
The tools you need to make a viewfinder are cardboard (acid free if available), a good knife, two big paper clips and a ruler. Use the ruler and the knife to cut two evenly shaped L pieces of the acid free cardboard. Now the size is up to you but the recommendation is 20 cm (8 inches) for the arms but bigger is fine you can shorten them later. Make the L shape about 5 cm (2 inches) otherwise they will bend when using them. Also to keep in mind the wider the frame of the viewfinder the more unwanted distracting objects are blocked from view.
Use the two big clips to join the two L-shapes together so they form your frame.
You can adjust the frame by sliding the L-shapes horizontally or vertically as you like and as you need. Through the constructed opening you can find the best area to paint by moving it from area to area.
In other words place your viewfinder on your photo (or hold it up in front of the scenery and look through it). Keep adjusting both L-shapes until you have found the composition you like.
This is all there is to it on how to make a viewfinder, it is very simple but very helpful. It is also a good idea to make more then one view finder, make a small one to use on photos and larger ones are perfect for spotting the best composition when you are painting "en plein air."
Now with this watercolor painting tips I will leave it up to you and make the greatest watercolor paintings you can do.