A base N number system has N digits. The process of conversion of N based number to decimal and vice versa is very similar to the binary number system process we just saw.
The only thing that changes is that 2 is replaced by N.
So, if the N based number system representation is
An An-1 ... A2 A1 A0, then the corresponding decimal value is
A0 * N^0 + A1 * N^1 + A2 * N^2 + ...
The reverse process is also exactly similar. We keep dividing by N, keeping track of the remainders as we go.
Let us say our number is
N = 7.
1 R0 --------- 7 | 7 R2 --------- 7 | 51 R0 --------- 7 | 357
Thus the number converts to
1020 in base 7.