Step #1 - Make it both expected and unexpectedEdit
People will always expect a screamer in a situation like a screamer vid (it's even in the name), so try and throw people off by making many moments where people will expect the screamer. Some good examples are;
The screamer you use should be unique, or rare. Make your own scream sound effect and use an image that is rarely, or ever, used.
Avoid over-used images such as the infamous Regan image or Anne.jpg or the eyeless woman.
Make the context unique, don't make it a "When you see it" or a "What's wrong" or a "I heard/saw a ghost" video; do something at least a little big original.
If you are going to make it a common type of video, give a few actual 'what's wrong' or 'spot the difference' images, to make it seem legit - picking out the actual errors.
If you give subtle / hidden hints or warnings - the screamer will become much better once people notice them on a rewatch/replay. Don't make it too hidden or luck based, it might as well not exist if that is the case.
A warning in the top line of the description of a YouTube video is also good enough.
Step #4 - Make the screamer introduce themselvesEdit
This makes a screamer more effective from a simple image popping up, also known as a Cheapscare nowadays, as people get a reaction just before the actual screamer happens - thus amplifying how much they get scared. This is used in many famous horror films such as 'Mama', which is one of the scariest films of all time in my opinion.
Jumpscares are the easiest way to get a reaction - along side shock. So try to branch out in scaring people. Easy ways of doing this is atmosphere, and expectation.
Many good screamer videos (such as the three previously mentioned in Step #4) are shot in Green-scale night vision, with a lot of space visible. Other good screamer videos are shot in black-and white (Real Demons Caught On Tape), limiting what you can see - technically.
There will be people clicking on your game/video expecting it to be legit, that's hopefully what you wanted, so they will not like it when the screamer appears - so they will most likely block you, downvote/dislike it, leave hate comments or even report it.
Always remain aware that this is most likely going to happen.
Good screamer videos/games should be at minimum 15 seconds long. Some videos can be way too short, thus way too sudden/abrupt, such as 'Short Screamer' and 'Folge dem roten Strich 2' some take way too long too.