Try to make the sound of gurgling water while your head is in normal position (i.e. not like real gurgling where you'd look at the ceiling)This way, the back of your tongue would automatically be a little curled up. What is produced should be close enough. You can practice it with German speaking friends online to see if you have got it right. Hope that helps.

In "zurueck" wird das "r" schoen gerollt. Die Zunge bleibt vorne. Bei "irgendwann" nennt man das "psuedo-r". Die Zunge wird zurueckgezogen und es wird nicht gerollt.