I guess, on a basic two channel sound card or interface, connect line out left to tape line in (any track). Connect tape line out (same track) to soundcard right line in.
Play back the file you want through line out left by routing it in your software mixer. Record it onto tape with the machine's monitor switched to "tape". Set soundcard's right input for record and start the whole thing.
Adjust record levels (in both tape and on digital recorder) to suit. You will probably want to overload the tape but not clip the soundcard input.
Do this for whichever digital tracks you want to add tape distortion to.
Now the taped track will be about a half a second or more behind the original track. Make a note of the exact time difference as accurately as you can. Then you can use that same figure each time you do the next step. Visually and by ear, move the track to the left until it aligns perfectly with the original track. This can take some fiddling including zooming the screen timeline up. Once you feel it is right in sync, you're done. You will have two tracks or files of the same material both the second one has been through the tape machine.
So it's no different to using any outboard effect using analog signals.
This setup as opposed to tracking everything onto tape and then dumping into digital would be my preference because you are being selective in which tracks you want to deliberately add tape distortion to.
Hope this helps.
