I've had some experience with starching. I keep a piece of muslin to lay over the ironing board for when I starch. It catches the spray and helps keep starch off the ironing board cover. Also, I use a mesh heat resistant cover:
https://www.walmart.com/ip/High-Temp...olor/553644369
Usually I'll starch my fabric while still damp from being prewashed, and I'll turn the fabric wrong side up on the ironing board to iron. Then I have to be very careful and use a light hand while ironing. Seems like starched fabric can more easily become stretched out of shape. If the iron is too hot it will burn the starch causing scorch marks, which will wash out. Sometimes the burn will go through to the fabric; I had that happen once. Hoping you just have burned starch and it comes right off.