Put the lid on; start with high heat, then when it boils reduce to medium heat and simmer for about 7 minutes.
Literal
Lid [object-を] do, beginning [topic-は] high-heat, boiled-[when-たら] medium-heat [to-に] do about 7-minutes simmer.
A cooking instruction using the natural sequence of te-forms. ~たら marks the conditional trigger: 'when it boils.' The progression 強火→中火 (high heat→medium heat) is standard cooking vocabulary. 約 (approximately) is a compact formal prefix. Bare dictionary forms (して, 煮ます) alternate between casual and polite in recipe style.