Add aux input to Audio Amplifier circuit
Impossible to see what's optimal because we do not know the exact schematic. If you are a beginner, do not dig anything on the board, insert a mixer to the input as already suggested by others.

The simplest possible mixer is to connect 2 signals to the same input through few kOhm resistors. You lose easily 50% of the input signal voltage, but that can still be fixable by cranking the volume upwards. I have used 3,9 kOhm resistors in series when two line level signals had to be connected to the same input. The signal sources gave about 2Vpp and less than half was needed, so the caused 6dB attenuation wasn't a problem. I made a cable which had the series resistors.

Using 2 different resistors you can get some control over the balance of the combined signals. But the sum of the resistors must be a few kOhms.

If you need continuous adjustability, you can use a few kOhm linear potentiometer instead of 2 resistors. The viper is connected to the amp input. A stereo potentiometer adjusts both channels. 

If you cannot understand what the text above means, do not try it. Get some local help!

A properly designed summing mixer with independent volume potentiometers and possible tone controls, too, gives much more flexibility, but the cost of home made 2 resistor cable is unbeatable.