I assume you're aware the option is under 'Look and feel' based on the way you asked the question.
There is no simpler way to use the function, but maybe you're better describing what Calibre did that Sigil didn't like. There are a number of features that are enabled by default during conversion that may be what you 'don't like'. If you described what it is maybe we could help you. At a guess you should disable chapter detection, page break insertion, and splitting on page breaks.