Maintain solid spinal alignment. Most people who are new to this will want to cheat the single-arm plank by creating cervical hyper-extension and tilting their heads up. This strains the neck and places excessive tension on the lumbar spine by causing the hips to sag.
Focus on keeping a tall and elongated neck throughout rather than tilting the head up or down.
How many reps can you do for two alternating exercises in 15 minutes? This is just a taste of how escalating density training works. It’s a brutal, but fun way to build muscle and improve conditioning.