Concept:
When using the conjunction \textit{lest}, it expresses a negative purpose and is followed by should + base verb.
The structure generally is:
\[
\text{Main Clause + lest + subject + should + verb}
\]
Step 1: Identify the main verb and subject.
Main verb = take
Object = light diet
Conjunction = lest
Subject = you
Verb after lest = should fall ill
Step 2: Arrange the words according to proper grammatical structure.
Final Answer:
\[
\boxed{\text{Take light diet lest you should fall ill.}
\]