Seashell, Pentacle, Tall man, old movies, Lamps ... it's all connected
Long post - skip the background if you know your history. Anyway, the film seems to retell the legend of Venus from greek mythology or at least it makes numerous allusions to the mythology.
Background: Mythology & popular culture
Venus or Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation. Aphrodite is derived from aphrós "sea-foam" and the goddess is said to have "risen from the (sea)foam”. She is said to have floated ashore on a sea shell. When she arose from the sea-foam she saw a Calla lily and overcame with a pang of jealously.
Venus (the goddess) is identified with the planet Venus. The pentacle of Venus (five edged star) is derived from the planets orbit around the sun.
Giants appear in Greek mythology and the most famous ones are the hundred Thracian Gigantes who waged war on the gods, including Venus.
The painting "the birth of Venus" depicts the goddess Venus, having emerged from the sea as an adult woman, arriving at the shore in a sea shell while a woman is holding out a blanket/rope for her to wear.
The film "Voyage to the Planet of Prehistoric Women" makes strong allusion to both mythology and painting as can be seen from the plot description: Astronauts landing on Venus encounter dangerous creatures and almost meet some sexy Venusian women who like to sun-bathe in hip-hugging skin-tight pants and seashell brassieres.
Their connection to It Follows
Seashells
Yara's e-reader and the chairs in and around Jay's house are all shaped like seashells. Jay is wearing a necklace made of seashells
The pentacle of Venus
Kelly & Jay look at a house decorated with the pentacle of Venus
Voyage to the Planet of Prehistoric Women
The movie is playing on the old TV
Calla Lily
The lamp in front of Jay's mirror is shaped like Calla lilies
Sea foam
Jay and Paul sit beneath a painting of sea foam as if Jay has just arisen from the foam like Venus/Aphrodite.
The birth of Venus
At one point, Jay wears a blanket and next to her is a chair shaped like sea shell as if Jay was Venus herself right after having stepped out of the clamshell and received the rope/blanket depicted in the painting
Giants
Jay is attacked by a giant/tall man right after being linked to a Greek Goddess. Giants waged war against the gods cf above