I would like to do some, but i dont now how.

Are you using galaxy? In that case you can do it with F12. If not, you can do try F11 which should create an ingame screenshot in your user's Stellaris folder (as described in the wiki).

pressing f12 will not do jackshit because Paradox didn't link their game into Galaxy

to take a screenshot in Windows 10 you press [windows key] + [prt sc] and this puts it into your defalt documents pics | screenshot folder... then you convert it into a shittty jpeg if you want to post it here because Galaxy uses the Google system... most other places allow png

Ok, thanks for help!

Some of the war i had in Imperator was so epic huge (like + 2,000,000 casualties both sides) that i would like to eternize them.