I never asked her about Ishtar, but my best guess is, it is possible Ishtar might be a different mask or aspect of the same entity. How similar their sigils look would have little to do with that, but that’s just a guess.
You don’t need to do any form of pathworking or chant demonic incantations. She isn’t demonic as far as I can tell. Different energy, entirely.
Just open the sigil and vibrate her name until she comes. You can vibrate, “Astarte, come!” as that works.
If you want to build an ongoing relationship with her, you can set up a basic altar for her. You can use a red candle, draw or etch her sigil into it, pour her a glass of wine (any wine will do), a glass of water, and burn a stick of lavender incense. Offer that to her on her altar for 24 hours, and do this once a week. In time you may want to sit with her daily with her offering.