First, even the most intense routines will take months to get visible results. PE takes patience, so if you’re feeling discomfort or pain you’re trying to hard or doing something wrong. Successful PE is more about how much time you’re willing to devote to it, rather than how intense of a force you use. I’m making a point of this first, because it is critical that you have the right attitude toward PE, or otherwise you’ll end up trying too hard, possibly injuring yourself, and becoming discouraged before ever seeing any results.
Generally, the length you stretch at is longer than your erect length, because you’ll be stretching while flaccid. Even if you start erect, after a couple minutes without stimulation you’ll go flaccid again, and your flaccid stretched length will usually be longer than your erect length, but it’s never less than your erect length. So the from the base of the extender to the ends of the extender rods, the length while uncompressed should be about the length of your flaccid stretched penis.
I’m guessing you just bought the extender and are completely new to PE; so you’ll want to start with a low level of tension first, and after a couple weeks then you can try increasing the tension. Fortunately the Jes extender has some markers on the rod to give you an idea of how much force you’re using (because a spring force is the spring constant multiplied the distance the spring is deformed, so the force is related to distance). I think they have the details on the exact force on their website, so you can look that up if you’re interested.
The standard extenders use silicone straps and/or tubes to grip behind the glans of the penis like a noose; but this means they’re compressing the tissue and restricting blood flow into the penis head. This can result either in a build up of blood in the head, or prevent blood from entering. Both are undesirable, so while wearing your extender you’ll want to occasionally check on the head to make sure it’s still getting enough circulation. The newer nooses are better at allowing circulation than the old ones, but eventually you will need to take the extender off to allow blood flow through the head. Once the head returns to it’s natural colour, then you can put the extender back on for another session.
For slippage, the answer is to wrap the head. The idea here is that the wrap will give the noose a higher friction surface so it can grip the head better. There’s a lot of tutorials on how to wrap, but a really simple one is apparently some paper towel. Just tear off a little strip and wrap it around once and tape it down with a bit of tape. Ideally the tape will be on the underside, so the noose grips the paper towel which offers better friction. The wrap shouldn’t be too loose that it slips off, and it shouldn’t be too tight so it cuts off blood flow. This takes some practice, but you’ll get better at it the more you do it.
The Golden rule for all PE though is if it ever feels painful: stop immediately. There will always be a bit of discomfort, but never pain. Certain kinds of discomfort are to be expected: With an extender, you might feel some discomfort where the base is pushing against your body. That’s normal, and you can always add some cushioning to help that. If you’ve been stretching for a long time without a wrap, then you’ll feel some pain from the skin where the noose was when you remove it. If you do feel any discomfort that worries you, stop what you’re doing and wait for the discomfort to go away. Then test to see if stretching your penis causes the discomfort to come back. If you don’t immediately feel the discomfort again then you can try stretching again but at a lower intensity. If you do immediately feel the discomfort again, then stop for the day and see how your penis responds tomorrow.