it's more or less like German ö or French œ.
You start by pronuncing the o of English "boy" then you push your lips forward as if you were going to give a kiss.
Do it in front of a mirror and after a few tries it will work.
In American English a barn is where you keep the farm animals. So that is very confusing for me. But I'm learning and I pronounce born like the word born whether the o has this little sign or not and it takes it. Usually on the third try. But it takes it.