To improve your bond level, you need to talk to one of them and tap on the "GIFT" icon as illustrated below:
After that, you need to select a suitable gift for them to acquire bond points. The example below is a gift that Magnolia might like:
After gifting them once, you need to wait 6 hours before the next gift.
Do note that the girls do dislike certain gifts and will cause you to lose bond points instead. Also, if you stop giving them gifts for a certain period of time, bond level will decay.