If you've already got the document saved, this is how I'd do it:
Insert flash drive.
Display the folder where the document is stored (e.g. 'My Documents'.
Make the window fit only part of the screen. (Click on the overlapping rectangles in the to right hand corner and then drag the edges of the window if needed).
Double-click on 'My Computer' (or 'Computer', depending upon which version of Windows you're using).
Double-click on the icon for the flash drive, to open a window displaying its contents.
If needed, also resize that window, so that you've got the two windows displayed side by side.
Click-and-drag the file from one window to another.
Job done.
--------------------------- OR ----------------------------
Insert the flash drive.
Open the document.
Go to File > Save As
Click on the downward-pointing arrowhead, at the right of the 'Save in' box.
Double-click on the icon for the flash drive.
Click 'Save'
Chris