Done! Now I'll just turn it down to a low flame so it doesn't boil till everyone arrives.
Literal
Doone! After-は everyone [subj-が] come [until-まで], boil-[not-ように] low-flame [to-にして] [emph-っと].
でーきた is the vowel-lengthened exclamation of 出来た 'done!' — the long vowel marks triumphant satisfaction. 沸騰しないように 'so that (it) doesn't boil' uses the ~ないように pattern 'so as not to / in order not to.' にして is the te-form of にする; the trailing っと is a self-directed verbal tic, the sort of muttered mark a speaker makes while executing a task — 'okay, and now...'.