Another way to do this, for "perfect" entropy calculation is to roll six-sided dice and write down bits.  Then convert those directly to a private key.

If you roll:

1:  write down "00"
2:  write down "01"
3:  write down "10"
4:  write down "11"
5:  write down "0"
6:  write down "1"

Yes, you only get one bit when you roll a 5 or 6, two bits for everything else.  Stop when you have written down 128, 160, 192 or 256 bits (whatever your preference is).  If the dice rolls are perfectly random, so will your resulting binary string.