I guess you want to convert 0.375, the decimal fraction, into a standard fraction? All you do is count the decimal places. There are 3, so you write 1 and 3 zeroes: 1000 and make a fraction 375/1000. Cancel this down until it won't cancel any further: 375/1000=75/200=15/40=3/8. So 263.375 is 263 3/8.
You can carry on and make it into an improper fraction; 263*8+3=2104+3=2107. So the improper fraction is 2107/8.