The easiest way is to tie a fixed loop in the middle of the rope (figure 8, alpine butterfly, bowline on a bight, etc) and then clip the climber in to that loop using two locking carabiners. Two carabiners are used here in order to avoid the scenario of a single carabiner rotating into a cross-loaded orientation during a fall and failing as a result.
However, my preferred method is to tie in directly to the rope, with no additional carabiners, using a bowline on a bight. This can be achieved by tied the knot using a long bight that can be passed over the climber's entire body instead of just being passed over the rest of the knot. This gives you a bowline on a bight tie-in with a very large loop, which you can feed back through the knot to reduce the size of the loop before climbing. You then need to expand the loop afterwards before untying. Those who don't know how to tie or visually inspect and verify a bowline on a bight should not attempt this, as incorrectly tying the knot can easily result in a knot (or lack thereof) that is of inadequate security or even completely insecure.
Another method for those more familiar with the figure-8 is to tie a figure 8 loop in a bight, just in the rope, not yet attached to the climber, making the loop large enough to fit over the climber's entire body. Then tie this into the climber's harness using a cow hitch (AKA lark's foot or girth hitch), by passing the figure-8 loop though the tie-in loops and then over the climber's whole body. This gives you a cow hitch tie in (which alone is insecure) backed up with a figure-8. The backup 8 will initially be way to far down the rope, due to its loop initially having been tied large enough to fit over the climber's body, but you can easily feed rope through to snug the figure 8 down against the cow hitch. As with the bowline version, you need to feed rope back through the knot to expand it before you can untie.
Edit: Images, as requested.
Bowline on a bight:
Incorrectly tying this knot can result in a knot that is inadequate or even completely insecure. Do not attempt this if you do not already know how to verify a complete bowline on a bight.
- Take a bight from the middle of the rope.
- Make a doubled turn about a metre from the end of the bight, with the standing end on the bottom of the turn. Feed the bight down through the tie-in loops from top to bottom. (Note: It is essential to get these two directions correct.)
- Pass the bight up through the turn, pull it all the way through, and pass it over your head.
- Bring the bight all the way down your back, and then step through it so it is hanging in front of you.
- Pull the bight through the knot until it tightens and forms the collar of a bowline, resulting in an bowline with an excessively large loop.
- Feed the excess loop through the nipping turn of the bowline. The result is a nice, tight bowline on a bight.
- In order to undo the knot, work backwards through these steps, feeding slack through the nipping turn to enlarge the bowline loop, and then pulling the collar out until you can step through it and pass it up your back and over your head.
Note that you can tie this knot in the opposite orientation, making the turn with the standing end on the top, and instead passing the bight through your tie-in loops from bottom to top. You would then pass the bight over your body in the opposite direction, from feet to head. The important part is that the bight is passed through the turn in the correct direction, as getting the direction wrong will result in the bowline collapsing into a girth hitch, which alone is not appropriate as a tie-in knot.
Girth hitch with figure-8 backup:
- Take a bight from the middle of the rope, and tie a figure-8 about a metre from its end.
- Feed the bight through your tie-in loops from the bottom up to the top.
- Pass the bight over your head, down your back, and then step back through it.
- Pull it tight, and you now have a girth hitch with a figure-8 backup that is a long way back up the rope from the girth hitch.
- Feed the rope between the girth hitch and figure-8 through the figure 8 to bring the two knots close together.
- Pull the figure-8 snug, and you now have a girth hitch neatly backed up with a figure-8.
- To untie, reverse the steps, moving the figure-8 up the rope until you can loosed the girth hitch enough to be able to step through the bight and pass it up your back and over your head.