how do i add an overlay when i have a helmet on

Use overlay condition to check for armor in your helmet armor slot.

but how to do that? i did this: 

                                               Event trigger                                                                                                                                                                                           ^Return>{(get item from armor slot[0]) = (MYITEM)}

but this just adds the overlay permanently and if I use IF DO then the do remains empty as there is no show overlay block I can use

You have to do trigger procedure to do this. It would look like this. You can add triggers by image on up. https://imgur.com/z4WCIC8

but where did that connecting red block come from where you connect the item and the slot?

its from the Logic tab

It doesn't work

Overlay condition procedure that returns true or false whether one has armor on definitely works