Connect two XLR cables to the stereo outputs of your tape recorder and connect them to the line input of your audio converters (Apogee, Lavry, UA, etc.). From the converters, connect the XLR outputs to the inputs of your computer or digital recorder. (See Reference 1.)
Open your recording software and set the bit rate to 24 and the sample rate to 96kHz. Unless you plan to make surround sound recordings from the tape transfer, there's no need to go beyond 96kHz sampling.
Using your recording software or other digital recorder, set two tracks (or one stereo track) to record mode but leave it paused. You're just going to set levels at this point.
Cue the tape on your reel-to-reel and hit play. Adjust the outputs to read slightly in the red on the VU meter at the highest point, but never beyond +4. If your recorder has an automatic mode try that as well. While the tape is running, look at the levels on your digital recording software. It should read well below zero DBFS, probably between -18 and -12, depending how it's calibrated—plenty of head room.
When the levels look correct, rewind your tape to the beginning. Start the digital recorder first, then start up the tape deck again to record your analog material to digital. When finished, hit stop on the tape deck first, then hit stop on the digital recorder.