Find the largest power of 16 (i.e., 1, 16, 256) that will go into Dec.

If the largest power is, say, 16, and it goes into Dec 4 times, put a 4 in the sixteens place.

Subtract the largest power of 16 that will go into Dec giving a remainder.

Find the largest power of 16 that will go into the remainder and repeat the above steps until you get to the ones place (16^0).

The answer is Dec.toString(16).toUpperCase()