In PS go to the IMAGE menu at the top then pull down to IMAGE SIZE and enter 640 for either the width or the height, which ever is the longest axis, so if it's a horizontal image then enter 640 in the pixel dimension box and vise versa for a vertical image and then enter 72 in the resolution box, click ok
NOTE, next step is very important. Under the file menu use the SAVE AS command and save your file with a different name so that you know it's a low-resolution image. You can set the compression around 7 that generally fine for a first time compression from the original. Just make sure you're not resizing and saving your ORIGINAL file as a low-res file.
In Lightroom select the file you want to export, go to FILE Menu select export. Choose the destination for the file for example desktop. File Settings , set format to JPEG, Color Space to SRGB Quality to 70%. IMAGE SIZING check the resize to box, then choose LONG EDGE set dimensions to 640 PIXELS, then RESOLUTION to 72 pixels per inch and then hit the export button and that should work
I always archive all my files before I ever touch them in PS or Lightroom, that way I always have a set of virgin files.
Let us know if you need any more help and good luck with the POTW.
Mark