Apply the thinset with a 1/2" square notch trowel- make sure to hold it at a 45 on the high areas & rake it up to a full 90 as you get to the lower areas. But first, mark or measure a height so you know how far the marble has to be set to give a full bed on the back.

Thjere is another way:

Marble can rather easily be ground down (it's soft as stones go) with a grinder or sander. This fact offers the alternative of some filling and some grinding to minimize the gap to be filled with adhesive- which has little sheer strength.

This allows you to grind then test-for-gap....not unlike a dentist with his carbon paper for contact.

In reverse, you can too grind off the high points of the concrete curb with a 4" grinder or circkar saw and a masonry blade. (have a vacuum handy for dust)

Even, continuous, support is the aim. No (structurally invisibly) span exceeding the (low) bending strength of the marble. Reducing the gap before filling is best.