Nominative plural : Masculine nouns:
Only masculine nouns ending in -ā or -ā̃ change in plural. These are the same nouns which change -ā to -e and -ā̃ to -ẽ in the oblique and vocative case in singular:
लड़का laRkaa , कुआं kuā̃ > लड़्के laRke, कुएं kuẽ
There are exceptions, like राजा raajaa, पिता pitaa, चाचा caacaa. Those words do not change in nominative plural.

Feminine nouns:
Feminine nouns in -ī, -i, -iyā change it into -īyā̃ : लड़की laRkii > लड़कियाँ laRkiiyā̃.

The other feminine nouns add -ẽ (including some nouns in -iyā).