Key design elements in my putter:
1. Round grip - 3"
2. heavier than standard (365 grams +)
3. counterbalanced
4. black head - no markings (no dots, lines etc.)
5. slant hosel
6. milled face
7. smaller than standard mid-mallet head
8. perimeter weighted
9. BGT shaft
(treatment for OCD on this subject)
Keep it simple with the "rope drill".
The most basic fundamental of all is that the golf swing is a rotational move of the body core, with the hands and club following that rotation through the impact zone.
Hope that helps.