Frosting on a glass surface to have specific patterns
TL/DR

Use a sponge and cut it to shape.



Loved an excuse for a quick experiment. So, the patterns in the video are made from an Epson salt solution with the following ingredients per the linked video.


  
  1/2 cup of hot/boiling water
  1/3 cup of Epson salts
  Teaspoon of dishwashing liquid
  


I tried a few different techniques to make specific patterns. The basic idea is to control the amount of liquid applied. Too much and it will bleed and drip down.

Napkin snowflake



Picture taken after solution was applied

So I wanted to try something that would hold liquid and not tear easily after it was cut. Most common napkins are frail so I used a heavier duty one. Any absorbent cloth would work but napkins are easier to come by. 



Tried to keep my face and dirty kitchen out of the picture. You don't need more detail to know it looks awful

This didn't turn out quite good at all. It was hard to get the right amount of liquid applied and keep the detail. Only my first attempt mind you. Read onto sponge for and improved version of what I was trying for here. 

Sponge



This is where I think you will get the most out of this project. This sponge I used for testing is perforated as you can see. I wanted to test fine details with this application. 



Look at the detail!

The level of detail retained from this application was next to perfect. I would highly recommend sponges for your application. You can pick some up cheap and cut them to shape. This will make it very easy to get applied control and maintain shape. 

Napkin ball

I know you are likely looking for more detail oriented designs but for completness sake I wanted to make larger items so I just balled up a napkin to make a heart design. This way my wife should not get made when she sees what I have done to the kitchen windows.



 I jumped the gun for the photo but it did eventually fill out nice

This worked as well as I would have hoped. The shape I made is what appeared on the window. It did bleed from the bottom so perhaps too much liquid was used. Easy to clean after the fact though with some water.

Fingers

Much like in the video using fingers would be a useful tool as well. In the video it is shown drawing after the solution was applied to the window. The difference with me is I used my finger to apply the solution to the window surface. 



Expected results here as well. Design showed well except where some bleeding occurred. I made a happy face which ended up looking more like a horseshoe. Other pictures are a star, the characters A&amp;C and an isometric box (that is not in the picture at all).



I recommend the sponge for sure because it helps with easy controlled application. Once you get a feel for the amount of liquid you can easily get the design you want without the salt solution creeping.