Understanding anti aliasing filter
From your statement, i understand a discrete-time downsampling of a sequence $x[n]$ already converted to digital, without aliasing, previously.

So to prevent any aliasing on the downsampled sequence $y[n]$, you should first apply an anti-aliasing lowpass filter to the input sequence $x[n]$. The cutoff frequency of the lowpass filter is given by the downsampling ratio $D$ and is:

$$ \omega_c = \frac{ \pi }{D }$$ radians per sample. The gain of the lowpass filter is one.

The following Matlab\Octave code designs a linear phase, type-I, FIR lowpass filter of odd length $L = 2K+1$ with  a group delay of $K$ samples :

D = 8;     % downsampling factor
K = 256;   % group delay of FIR linear phase filter.
h = fir1(2*K, 1/D); % FIR, LPF, Type-I, L = 2K+1, wc = pi/D