How do I sew a thick layer to a thin one?

Joining unequal layers fails in two ways: the foot tips, and the feed favours one layer over the other.

Put the thicker layer underneath where possible, so the feed dogs grip it directly. The feed dogs are stronger than the presser foot at moving fabric, and giving them the difficult layer helps.

Match the needle to the thickest layer, not to the average. A needle that will not pass through the thick layer breaks; one slightly too heavy for the thin layer only leaves a marginally larger hole.

Keep the foot level. If the step is significant, support the low side with a folded scrap or a hump jumper. A tipped foot stops gripping, which is when the feed fails and the needle deflects.

Lengthen the stitch slightly — around 3 mm — so the feed dogs take a bigger bite.

Use a walking foot if you have one. It drives both layers, which is exactly the problem being solved.

Reduce speed. Almost every technique for difficult sewing comes down to going slower, and this is no exception.

And check tension afterwards on a test seam of the same combination. Uneven layers change how the thread locks.