I don't understand where exactly is the problem? It's very easy and straigtforward to do base conversion: you do it as you would by hand.
- divide the number by base
- write down the remainder
- repeat the process with the integer part of the division
- stop when you reach zero
- the remainders in reverse order give you the digits in base
Example:
1025 (decimal) to base 15:
1025 / 15 = 68 , remainder 5
68 / 15 = 4 , remainder 8
4 / 15 = 0 , remainder 4
The number in base 15 is 485