To merge data from multiple columns in Excel into a single field in your labels, you can utilize the 'Insert Merge Field' option in Word during the Mail Merge setup. First, manually insert placeholders for the data you wish to merge into one field on your label. Then, by concatenating these fields (using the '&' symbol or through the 'Match Fields…' dialogue), you can customize and structure your address layout or any field to include multiple data points from your Excel sheet. For example, creating a full name from separate first and last name columns or combining street address and apartment number. This flexibility allows for personalized and precise control over the information on your labels.