Step 1
Whisk yogurt, lemon juice, olive oil, ketchup, garlic, red pepper flakes, salt, cumin, black pepper, paprika, and cinnamon together in a bowl.

Step 2
Place chicken thigh halves into the yogurt marinade and coat them thoroughly on all sides. Cover bowl with plastic wrap and refrigerate 2 to 8 hours.

Step 3
Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Step 4
Using 2 skewers for each kebab, thread half of the chicken thighs onto each pair of skewers making a fairly thick "log" shape.

Step 5
Place kebabs on grill. Do not try to turn them until they begin to unstick from the grill, 3 or 4 minutes. Turn kebabs and grill the other side 3 or 4 minutes; turn. Continue cooking and turning until chicken is no longer pink in the center and the juices run clear, about 6 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).