How do I delete imported photos from my phone?

It will depend on your system version on the iPhone or the Mac, if the "Delete items" option will be shown below the toolbar in Photos, when you connect your iPhone.

If you can see "Delete items", click it, before you select the items for importing, as described here:

Import photos from a camera or phone using Photos on Mac – Apple Support (UK)

It will depend on your iPhone model aand system version, if the option is available, however:

1. 1. "If you want to delete photos from the camera after importing them, select the “Delete items” tickbox. 
   2. This option does not appear for all cameras."

But you have to do it, before you start the import from the iPhone. This is risky, because the items will be deleted from the card, before we can check, if there have been transmission errors. The photos will be deleted from the iPhone, sight unseen. We cannot repeat the import, if Photos should crash, when indexing the new photos in the library or if some photos are showing a corruption.

I stopped using this feature years ago, after having seen in these forums, that quite a few users have lost precious photos to "Delete after import" or "Delete items". I am now erasing the imported photos from the device only after the import has finished, after I have looked at all originals, not just the thumbnails, and I am having made the first backup copy of the imported photos.