This is what I do with all my fosters:
I put them in a harness. I use a regular harness because they are small, but if the dog is large I'd use a front clip harness. I use a 20' lead, but have been able to use a 10 or 6' with a real tiny dog, at least to start. Go to a place with a little room (I use a field, but you can use a park) start walking. Head off like you have somewhere to go. Abruptly change directions without warning the dog. Presumably, he will be surprised when he feels the pull of the leash. Unless he is in full chase mode, he will follow you. If you have a small dog, you can walk directly behind him but in the opposite direction. If he is too large, make sure you are perpendicular to him (looking at his side rather than his tail). When he comes to the end of the leash he will be pulled off balance, giving you better leverage, and he'll suddenly find himself following you. Repeat over and over. You can use a triangle as your path, a square, a zig zag, just keep moving. You are also being trained, and as you work out the details, you can make sure that when he a) comes to the end of the lead; and b) gets out in front of you, that you change directions. You shouldn't be dragging him, if you are maybe you are walking too fast. If he puts the brakes on, you can either walk real slow so he's getting just a bit of a tug, or you can wait him out. Some dogs figure this out in ten minutes, others 25 minutes, but I just trained a puller who'd been pulling for about a year, 5 days. When he's figured it out and is paying attention to you so that he's never getting tugged by the leash, you have accomplished step one. Keep working him on the 20' lead a few days, then switch to a longer lead. At this point you can work on sit and down at a distance, and the recall.
If you were only going to walk him on a long lead and only care about him not pulling, you are there, but you should really train him to walk beside you. It is a safety and management issue. There are a few methods you can use, one is treating (or click and treat) whenever he ends up by your side. Another is to work him at home and extend the training on the long line walks. If you will mostly be walking him in an urban area (I assume using a park for the long line work) you can move to a 10' lead around the neighborhood, he will quickly get used to the shorter leash and learn that in the area with houses he must stay closer. On the 10' work on recall, down, sit, heel.