Great advice from Beanie
The other risk with a stretchy in a back carry is that if your LO decides to push away from you they may be able to push themselves out of part of the wrap.
At 15lbs you can keep using a stretchy but you will find a woven to be far more supportive. By 10lbs there is a noticible difference in the support and comfort. You have the time to debate and decide what suits you best, you don't need a woven that will arrive tomorrow but you would benefit from it as soon as you got it
You don't get baby in to a back wrap, you put baby up and then wrap round them. Maybe start just by practising getting him on your back and off again without trying to wrap. The way I am most comfortable with is puting DD on my hip, reaching round my back with the opposite hand to support her so that I can get the arm beside her in front of her then shuffling her into a piggyback. She loves this!!! You can also cross your arms, pick him up by having a good hold on the top of each of his arms and lifting him over your shoulder - as you uncross your arms he will end up facing the same way as you and you can lower him to the height you want. I seem to be hit or miss with this - I either get it right or I tangle us up
While baby is on your back you keep at least one hand on him at all times. Lean forward until the wrap is round him securely.
Have a look on videos on you tube - lots of people have filmed how to do carries and I have found that really useful. Good back carries to start with are ruck and secure high back carry (SHBC) as with this one LO is secure quickly and then you don't have to worry as much about holding him while you complete the wrap.
So, one wrap and only one. I ask myself this from time to time and usually the answer is Storchenwiege Leo. Not my prettiest (I have it in natural - it's not ugly, but I have some gorgeous colours) but sooo forgiving if your wrapping isn't perfectly done. I would recommend getting a size 6, which is 4.6 metres. In this length I currently have a Didymos natural Hemp which it has been suggested to me is even better but I haven't given that a proper trial to say. I also have Didymos Pfau, which would be up there with Leo and will be thinner in the summer. I have Blue Bears (dyed eggplant) and black Fische, both by Didymos (I like animal wraps) and they are very sturdy but have less bounce than leo or indios. That doesn't make them less supportive though. I have Earthy Rainbow by Girasol and although not my most supportive it is supportive and I think it is sooo pretty. Don't underestimate the importance of choosing a wrap that you like the look of, after all you will be wearing it a lot
I have a Calin Bleu that we used all last summer but I doubt we will use it this summer. It was brilliant when she was little but by 16lbs it was less comfortable. I have had and sold on didymos cashmere Silk Ellipsen (soo expense, handwash only, basically I was scared to touch it in case I damaged it), Didymos sik indio (it caught on my watch and it took me ages to fix the pull ), another Calin Bleu and another Girasol which I didn't need as I preferred the colours I kept. I don't have a hoppediz wrap but I do have one made in to a wrap conversion and it is fantastic so I imagine it would be as a wrap too.
I hope this helps!