Final answer:
In the procedure for making lemonade, 'Procedure', 'First', 'Next', and 'Then' are the signal words that guide the reader through the steps.
Step-by-step explanation:
The signal words in the procedure for making homemade lemonade serve the purpose of guiding the reader through the sequence of steps in the process. The identified signal words from the passage are 'Procedure', 'First', 'Next', and 'Then'. These words help organize the instructions and clarify the order in which actions should be taken, similar to how instructions for dissolving a solute into a solvent are presented in scientific procedures.