# Do not let steadfast love and faithfulness ever leave you

"Love God without ceasing and be faithful to him". The writer speaks of "steadfast love" and "faithfulness" as if they were people who could leave someone. 

# tie them together about your neck

"display them proudly like one would wear a necklace". The writer speaks of faithfulness and trustworthiness as if they were objects that a person could tie around the neck like a necklace. The image suggests that these are valuable things that the person displays outwardly.

# write them on the tablet of your heart

"always remember them, as if you had written them permanently on a tablet"