directionally challenged

[lmforgues]

Hello, I am attempting to knit after many years, was not great even then. Sooo, I am having trouble understanding what the directions mean. The pattern is berroco, the truman vest. Row 2 explains completion of the ribbing. Then after the purl row, it says "decrease 30 (36-38-44 etc) sts across 71(81-91-99 etc) sts. Work even in St st until piece measures 4 (4 1/2, 4 1/2, 5 etc) from beginning, end on WS."

I understand the abbreviations but do not understand what it means to decrease a certain number of stitches across a certain number of stitches.

Please help! It's even difficult to explain!