There are lots of ways to meet new women, places of shared interest, through friends, at work and so on. Be interesting and be interested in them for a start. Once you've hooked them into a conversation and want to carry things on either offer to take them somewhere else or get their contacts if that's not possible at the time. Everyone uses LINE and getting that contact is usually not difficult. You can use this to follow up,tell them how much you liked your conversation and offer to take them somewhere to continue.
Nanpa is simply going to an area where you know a lot of women will be around like places with lots of women's clothing stores and cute cafes and then randomly approaching one you like the look of. See those groups of guys with fancy hair and nice clothes hanging around on streets in Shibuya? They are probably doing that. It's about being confident and not being afraid of rejection because you can move on to somebody else after. Smile and approach, say hi and ask what they are doing then make conversation from that. Introduce yourself and get their name all while keeping them talking. Eventually you'll move the conversation with asking if they'd like to go somewhere with you for lunch or drinks. If it's on an evening at weekends and you offer drinks be careful of the young women who kinda counter nanpa to get a free night out lol. There are attractive women who seek out guys doing it so their night out is paid for and they often will make their excuses and bail before you got a single thing out of it. You need to keep things moving at your pace and don't get yourself left with a check when she decides she has to go meet up with her friends or something. You want her to keep having fun being with you and keep building sexual tension but you can't outright say you want to go somewhere and sleep with her at any point. I usually have things more or less planned out with how I'll set up the night and have it end up in a hotel.
Nanpa is always a gamble though, it might take a while just to be able to get women to randomly take you up on lunch and then getting them in bed after that can also go completely wrong. You could have a great night out,get all the way to a hotel and suddenly she becomes fussy and pulls away lol. At that point you probably can't do anything to convince her to sleep with you and you have to give up. I've read advice that it's better to lead her to a hotel and walk in without telling her you intend to take her to one but I've had about the same success/fail ratio in saying I want to go somewhere with a bath/bed because I'm tired without explicitly implying we will have sex. Even if they know they want to sleep with you they might back out if you ask them directly.In Japan it seems like women want the illusion of it just happening without intending it because they don't want to seem easy to men they sleep with. Or at least they don't want it to look like they are easy to people they know by them learning they agreed to sex and went to a hotel with a guy they just met.