Windows Scientific calculator: Start as "Dec", type in your decimal number then change to the "Hex" option.
Longhand:
0 = 0
1 = 1
2 = 2
...
8 = 8
9 = 9
10 = A
11 = B
12 = C
13 = D
14 = E
15 = F
Multiple digits gets a little more interesting . . . but pretty much the same as decimal's base 10, you just need to think in 16s when carrying digits and such . . .