To start the sans route you have to go Eeep when you first meet him, then romance him every time you meet. For asriel route you have to flirt with him twice in the ruins then stay on the friendship route until labs, then pick every option with a heart next to it. Wingding you need to stay on the true ending friendship route until you get to the castle. You’ll get a choice to pursue him there.
Full walkthroughs will be posted this weekend.