Converting each digit in an integer to its respective ASCII characters
To convert a whole number to a char array in one go use the itoa function. That sounds like what you actually need.   

To convert one digit from decimal to ascii just add '0', the ascii value for a 0. If you just want to try to understand what's going on, it's something like this.   

char result[4] = {0};
int num = 345;

int digit = num % 10;  // get the 5 off the end
result[2] = digit + '0';  // convert to ascii and store
num /= 10;   // get rid of the 5
digit = num % 10;   // get the 4 off the end
result[1] = digit + '0';  // convert to ascii and store
num /= 10;   // get rid of the 4
digit = num % 10;   // get the 3 off the end
result[0] = digit + '0';  // convert to ascii and store

Serial.print(result);  // prints "345"