Start with social media. You can try to make linkbuilding on Pinterest and Reddit, for example. After that make more external links on forums with high domain authority (DA). You can buy links or you can try making crowd marketing. I prefer to outsource it. Last time I ordered links from Crowdo company, and they’ve made a lot of high quality links. I don’t know how they do that. I tried to do it on my own, but I’ve failed. For some reason they did ban me as a spammer or something like that.