how to ?

First thing, try to remember what you did last. Or find your last backup that works and see what has changed since then.

If the CTD happens when you go into a particular area, try deleting things you added to that area. Delete half of them, see if there's a problem; if there is, undelete those and delete the other half; if there isn't, undelete half the things you deleted and see if there's a problem; and so on.

Define "instant". The instant you start the game? The instant you leave your house? The instant you drop a shield? etc

I think he means a crash to desktop that happens immediately, without warning, and which leaves no papyrus log messages.

well yes , playtesting my mod going around all of a sudden skyrim crashes and closes ...

How can I do to understand what's causing the issue?