Here are the steps for how you could combine your Muscle SpikerBox with Arduino Uno:

1. Solder one red wire (+) for the positive and one black (-) for the negative side to the battery terminals. It would be best if you could get the power supply cable for Arduino Uno and cut the side which goes to the 9v battery. The one from the link below.

https://www.ebay.com.au/itm/5PCS-T-type-9V-DC-Battery-Power-Cable-Barrel-Jack-Connector-for-Arduino-New-/191736126629

The reason is to power two boards with one battery.

Now this option could cause separate grounds problem, so it's recommend to have one battery per board.

2. Next, find the AMP2B label on the board and solder a male header onto it. It could be also found in the schematics below.

https://backyardbrains.com/products/files/Muscle%20Spikerbox%20V1.21.pdf

3. Then, that header you'll connect with A0 analog pin to your Arduino. The reason why it goes to the analog pin instead of the digital one is that the signal coming from the AMP2B is analog and has sine wave nature which will go through A/D converter on the Arduino Board.

4. At the end define the A0 port as the analog input in your code.