Replacement battery not working

My phone died a year ago, woke up in the morning and it wouldn't turn on, but would get warm when left to charge. I figured it was the battery as it wasn't holding much of a charge anyway. I just put in a new battery in hopes it would work for my daughter to use it. I haven't fully sealed it back together, as I wanted to check it first, it's still doing the same thing. I've never replaced a battery before, so maybe I did something wrong?