You need a splicing block, preferably an aluminium one. You will also need some single-sided razorblades (aka single-edge razor blades). These are usually steel so be careful not to magnetize them.
If you have problems finding some, they are often used in window scrapers.
Finally, you will need some splicing tape. I did try using sellotape instead, but it was not pretty.
It may also be worth getting a chinagraph pencil while you're at it.
The process is basically tape editing. If you need a loop for a Watkins copicat or similar echo device, just cut off a segment of tape approximately the same length as the original loop, and splice the ends together. It's too long, carefully peel back the splicing tape, cut some more off and resplice. Some editing blocks have multiple angles of cut - make sure you use the same one on both ends so that they match. Try and get the edges to line up without a gap - especially if you're using it for a tape echo.
If you're making an audio loop, the editing is a little more involved. You would use the deck's edit mode to find the start of the sound and mark the spot where it begins with a Chinagraph pencil.
Then you would jockey the tape (usually by hand) until you find the end of the loop, and make an
other mark. Then you'd cut both the marks and splice the loop together. It will usually take several tries, especially if you need the loop to match the tempo of an existing portion of song.
For instance, when I did the 'Endless loop of strife' in my recent song 'The Murder Game', dubbed the entire line 'An endless loop of striiife' onto 1/4" and then made a loop of just that one word.
Getting it to match the tempo was a problem and I had to get a few more fragments of tape to get it to match.
...several attempts.
The Radiophonic Workshop used to do this mathematically, they would use the 15ips tape speed and the tempo as a basis and measure out the length of tape they needed to cut. And maybe if I'd thought about it I should have done that too.
EDIT: Oh yeah, and once you've got the loop, playing it can be interesting.
Some decks really hate doing it, the A807 has a tendency to suck them into the mechanism which is bad, and some of the really, really high end decks like the ATR, Stephens or odd ones like the Lyrec FRED cannot do it at all because they don't have a capstan and use the reel motors as the tape drive.
In most cases you will have to defeat the tape-break sensor somehow. On TASCAM decks it's usually just a matter of holding the tension arm in place (maybe with an elastic band), for Studer derivatives there's a photosensor so you need to stick a post-it note or something over it.
In all cases you will need to tension the tape or things will get ugly. For a long tape loop like the ones the BBC did, or the 32-bar drums and bass loop used in early Cure albums, you use microphone stands, one or as many as you need and if it's big enough you have other people tending them.
My loops are usually short, about a foot or two, so I usually use pieces of bric-a-brac and a NAB hub or a roll of sellotape or an empty spool as the guide. I pile them up above the tape head cover. I add things to the pile until the tape is approximately tensioned. If it's too high it won't run, if it's too low it may escape or cause the pitch to fluctuate.
Here's a video which shows it being done:
STUDER A816: How to create a reel-to-reel tape loop - YouTube
And here's the original tape loop from 'Money':
Pink Floyd : Money in Studio VERY RARE!! - YouTube
...looks like they mic'd the audio on that video for some reason, but you get the idea.
