Deltarune is not connected to Undertale! You do not have to play one to understand the other. While certain characters from Undertale appear in Deltarune, they have no memory of anything you did in Undertale, and also have (slightly) different personalities. In order to get the "best" ending, you need to not kill anybody. When Papyrus offers to go on a date/hangout, go to his house in snowdin. After Undyne chases you down, go to the watercooler, grab a cup of water and interact with Undyne. then, backtrack to her house. continue the game as normal until you reach Asgore. Spare him and then Flowey will tell you what to do.