Professional-level reel-to-reel tape decks generally have two or four tracks. Two-track machines, especially at 15 inches per second recording speed, produce the best home-studio recordings. Two-track recorders provide more physical space on the tape, and the faster the speed, the better the quality the recording and the less tape hiss. You can plug instruments or microphones directly into the tape deck or go through a mixing board. A mixing board will give you more control over volume and tone and will allow you to record more than two things at a time. Always use fresh tape, not erased, for your recordings. This will prevent sound glitches, plus it will save all the variations of your recordings.
Most pro-level open-reel recorders have three heads: erase, play and record, in that order. The erase head clears the tape before it gets to the record head for a clean recording. If your machine has sound-on-sound capability, it works by turning off the erase head and allowing what you've recorded on the tape to play through the playback head so you can hear it to record new material with the old. This is a good technique for those who are one-man bands and record all the parts---guitar, bass, drums, keyboards, vocals---themselves. However, some recorders create a slightly off echo, so using two open-reel recorders works better. To do this, have your first recording on one deck wired into the second deck through line or microphone inputs. Listen to the first recording in headphones and play the next piece of music along with it while recording to the second deck.
Three-head decks also allow for the creation of built-in echo. If you record directly to the deck, wire the line-out jacks to the line-in jacks. Because of the separation of the playback and record heads, this will form an echo of what you're doing live. Be sure to watch the VU (volume unit) meters, however, or the echo can get away from you and overwhelm the recording. You can hear what's happening in headphones, so if the echo begins to swell---and stops sounding like music---you need to start again.