We want to divide the number of favorable outcomes by total possible outcomes.
How many total outcomes: 6*6=36
How can we get a sum of 8?
4+4 (four on both)
5+3 (five on first, three on second)
3+5 (three on first, five on second)
6+2 (six on first, two on second)
2+6 (two on first, six on second)
Since there is no seven face on a standard die, those are the only possible favorable outcomes.