The main trick of PMax is its automation. It's a cool type of campaign that I use both for promoting services and for online stores with product advertising.
But you have to first prepare all the materials and track the necessary conversions on the site and import them into the ads, because pmax is optimized ONLY on conversions.
Prepare banners of all necessary sizes, 7-10 headlines, 5 descriptions, a couple long headlines. Preferably a video as well. And launch the campaign. Let it learn for a month on a stable budget.
Just be careful with placements where ads shown. Check in the reports, on which sites your ads are shown. Since pmax is shown both in search, and on the display network, in youtube, gmail.
Good luck with the launch!)