I was lying in my room and my dad (has ABSOLUTLY no idea what he is doing when it comes to keeping a tank [he wanted to add fish the first day we filled the tank])

comes in with a bag of two mollies (one dalmation one black). He told me he went to the fish store and they gave him these fish for free to put in our tank (this is all after my first mollie died [i still cant find the second one]). Right now the two new mollies are still in there bag because i dont want to add them in. But i know i shouldnt really keep them in the bag for long.
Is it ok if i add them?
and how EXACTLY do i do it? I know i have to slowly add in water from my tank for 2 hours, but how much water do i add etc...
please help me. im in kinda a sticky situation.