The Propex I bought came with a compression 90 fitting that screws onto the inlet nipple. I purchased a flared one end thread the other 90 & did not use the Propex supplied compression 90.This.
Top of the list.
Use copper lines and flare fittings.
NOT compression fittings. Compression is for water and non hazardous only. They vibrate and leak.
Don't listen to anyone that says they used compression fittings "and never had any issues".
If you don't know how to flare or don't have a flaring tool, just buy or rent one, or ask a plumber to run the lines.
A plumber buddy might do it for a few beers.
Yellow gas tape on the threaded end & no tape on the flare end.
Im not a fan of compression fittings either.