Download and save the file on your computer, then extract to a new folder.

Locate the Stardew Valley game folder on your computer's hard drive, then locate and open the Content folder.
Always back up your game files before using mods.
Inside the Content folder of Stardew Valley open the Characters folder and replace Baby.xnb with the downloaded Baby.xnb file.
You will now have a little baby Sam that plays the guitar!

Thanks and good gaming to you! -Pecuniam Art