How do I make diamond mullion widow decoration?

Hi Cj, There are several different styles of muntin's. Here's a how to link with a simple idea: https://thechelseaproject.com/how-to-easily-diy-custom-muntins-for-windows/ I think they call this one Prairie.

I'm sending this lin because it shows several styles: https://www.easyhometips.org/choosing-best-grill-design-for-glass-windows/

This how to video shows the most basic: https://www.youtube.com/watch?v=y0NXE6gpqr8

What ever you decide, have fun!

Which do you mean?? What is the difference between a mullion and a Muntin?

While window and door people usually use the term mullion to describe both vertical and horizontal members, the term transom is the most correct name of horizontal “mullions”. Muntins on the other hand divide, reinforce and join glass within a single window or sash frame. You could use Washi tape for the look easy,low cost https://www.google.com/search?rlz=1C1CHBF_enUS750US750&q=washi+tape+on+windows&tbm=isch&source=univ&sa=X&ved=2ahUKEwjLuNCJrbLiAhVewMQHHWWuAFkQsAR6BAgJEAE&biw=1366&bih=657 electrical tape-- https://www.houzz.com/discussions/quick-and-easy-faux-window-panes-dsvw-vd~599201

Try this: https://www.newpanes.com/howitworks.html