I'll try a real answer!!!
So, do you know how to operate the Boss drum machine thingy? If not, start there.
*waits*
Okay, now that you know how to operate the drum machine and have created the appropriate beats, let's get down to business.
Do you have a computer of some type (Mac or PC), that has been purchased in the current millenium? If so, let us know the specs, and here's why:
You are looking to create some sort of multi-tracked recording, whether you have the option of mixing later or not. In the case of recording guitar over created drum beats, and running the mix to a stereo recorder (like a cassette recorder), you are not going to have the option of mixing later.
The easiest way to go about doing all of this stuff would really be to buy an interface for your PC or Mac, and then record in that manner.
However, if you are hell-bent on simply mixing guitar over a drum machine into a stereo recorder, then at the very least you will need a mixer. Let's say something like this guy:
http://www.musiciansfriend.com/product/Behringer-Xenyx-502-Mixer?sku=631262
Essentially, you would plug the two 1/4" output jacks from the Boss drum machine into the L & R 1/4" jacks on channels 2-3 or 4-5, and then turn up the volume on the mixer appropriately.
Then, using channel 1, you would run in your guitar signal. I'm not sure what that might be - whether its a microphone in front an amp or an acoustic, or if its a guitar plugged straight in.
Then, after monitoring over headphones from the headphone jack on the little mixer, you would run the stereo Tape-Out to the input of your cassette recorder. Hit record, then start the Boss machine and play along.
That's the barebones way of doing what is nowadays considered one of the most crude multitrack recording sessions I can think of.
If you have a computer, all you would need to do is buy an interface, which can be explained as well, and then use multitrack recording software to do all sorts of fun things. Plus, the mixes you create won't be permanent, and won't have been recorded to something as lo-fi as commercial cassette tape.