totally depends on your tape deck, it may not even be possible.
That said, if it is possible:
Your mixer will have a "line out" or "rec out" RCA ouput.
Your tape deck, should have a "line in" or "rec in" RCA input.
Get yourself a good old RCA-RCA cable (male to male). Connect the inputs to the outputs.
Probably tell the tape deck to use the input signal instead of recording from the other tape deck (assuming this is a dual tape player).
Hit record.
There is the off chance that your input to the tape deck is not RCA but 1/8th inch (like a headphone plug). In which case you want to buy an RCA-1/8th cable (male-male) instead of an RCA-RCA (male-male).
To summarize. You need to find the output on your mixer, and the input on your tape deck. Get a cable that fits them together. Then tell the tape deck to record from that input.
cds