You can use excel to create genesets. The format must be as follows:

Column 1: name of gene set

Column 2: Empty, or you can use this column to store information about e.g. the source of the gene set.

Column 3 and onwards: id of genes belonging to a particular geneset

gmt file format

When saving the file, make sure you save the file as a tab delimited text file, and type the extension .gmt at the end of the filename. If you see in the file manager that excel has added .txt after .gmt, you must right-click the file, select Rename and remove the .txt extension.