Apple iPhone 5c Black screen and broken home button

I'm repairing a friends Iphone 5c after the phone was dropped. The screen is cracked and is black. I managed to get a picture on the screen by giving it a wiggle (suspect cable damage) and found also the home button does not work. Also there was some stains on the LCD so I reckon that it is stuffed. I'm waiting on a new screen to arrive but the problem is I now cant get the phone to power on at all and when i put it into the charger it "bongs" every 3 or 4 seconds. I have also checked to make sure the connectors are down inserted correctly.

I suspect that the issue will be fixed when the new screen etc arrives but my question is how do I proceed with this repair as I cant power the phone off before starting?

What damage may happen if I just do the repair?

Should I just disconnect the battery before starting the screen repair?

Thanks