This is not really related to Jekyll but...
- Good content! Not just nice one!
- Use meta tags, like description etc.
- Use title tags accordingly
- Add share buttons - sharing indicates something to google, as I have learned at customer site
- Add opengraph tags (for sharing)
- Structure your html nicely
- Make use of page speed tips: https://developers.google.com/speed/pagespeed/
- Create good links: readable (for the users) and short (for the users too). Makes your site more trustworthy and shareworthy, if there is good content
- Create a good mobile version! (Pagespeed tool above helps!)
- Consider AMP - not sure if it helps
- Use https - Google is said to use that as ranking factor
- No fancy JS, even when Google says it can render it. We have tested it at large scale and fance JS stuff which shows content is not as good as plain html/content
Could go on, but that's probably the best. People are saying keywords and bla, but I believe it's not necessary. Just write good content so people share it, and you are done.
Having done that all above, some pages of mine work really well and some don't. So it's also a matter competition. Again, when you have the BEST content out there competition will be left in dust.