Note: You will need a computer to do this unless you plan on writing out pages of calculations for just a few values. This will detail how to calculate a value of the analytic continuation of the zeta function:

First we will calculate:

for a finite (hopefully large) value of n so

We then have to calculate one value of

if s is a complex number we can use Euler's formula

because

we have

after this calculation we will have a complex number in the denominator

to get it to the numerator multiply by the complex conjugate

the resulting complex number has

this can simply be added to any values for n that were calculated before.
Now we calculate

This can be done with calculation similar to the previous one but noting that

and

finally multiply the two complex numbers together.