Attached is a sample flat file that i edit in notepad to change the field names and field lengths..
the way it works is you open the "flat file" in notepad and set your field names and lengths(sample below)
once finished, save the file..then from Alteryx, setup your input for the text file you need to parse, and when prompted choose to import as a "flat file"
Then, when you get to the screen below, select "Use Field settings.. " option. Select the folder icon to the right and then choose the "flat file" that you updated from above as your template.
Once you select your file, Alteryx will use the field names and lengths to parse your source file. (see below)
Hope that helps..