Headphone jack install in turntable

I bought a Crosley CR249 Keepsake USB Turntable a little while ago and I love it. The cabinet is all self contained; it has speakers built in. A major downside is that it doesn't have a headphone jack. I'm interested in installing one but I'm not too knowledgeable about circuits or anything. I have a basic knowledge and I can read a schematic fairly well, and I know how to solder, so if anyone can help me out, I'd love it. I figured I could just install a headphone jack that would bypass the speakers when it was plugged in, but I don't know how exactly to go about doing that (where to attach it in the circuit, what resistors I'd need to use, etc.) If you need pictures or anything, I can get them up here, too. Again, I would love any help I can get.

More information: the power consumption of the whole device is 12 watts. There are two speakers, each of which says 3 watts and 8 Ω. I thought they may be some useful information...