Question

She laid the table after she had finished the cooking. (Begin:When.....................)

Answer 1

The sentence will be written as shown below:

When she had finished the cooking, she laid the table.

Explanation

Whenever we have Past Perfect and Past Simple, we must remember, Past Perfect refers to an action that happened earlier. Simple Past is for the action that happened later. So as per the requirement (Beginning the sentence with 'When'), we will write the action that occurred first (had finished the cooking); and the action that happened later (laid the table) 

Answer 2

When she had finished cooking,she laid the table