That depends on the original word. I think most words ending in -e you just add the "r" to make it plural, i.e. kvinne = kvinner (woman = women) eple =epler (apple = apples). If it ends with anything other than -e then you add the "er", i.e. gutt = gutter (boy = boys) vin = viner (wine = wines). But like all rules there are exceptions that we just need to try to memorize.