Research the god/goddess you are wanting to evoke find out what correspondences they have: planetary,gemstones,incense,sacred animals, traditional offerings etc
Then as @anon69472442 stated build an altar around what they like and you can set up a traditional depiction of them as well. Doesn’t have to be anything fancy a statue displayed on your cellphone will work or print it out from online.
Many of them also have associations in the tarot deck as well so you can even add their tarot card onto your altar as “their sigil”
Check the forum and/or google because some magicians have created sigils for them that can help give you a shortcut or as another member stated above create your own after making the intial contact
You can also find incantations on YouTube that will translate the greek prayer into english subtitles although in my personal experience you don’t need to speak greek to get their attention but if you want that flavor go for it but you can call out to them in english from the heart and they will hear you