As soon as you start the new action, Photoshop will begin recording your actions, however you can stop and edit your recording at any time. At the bottom of the Actions palette you'll see a row of buttons, bearing icons that will be familiar to anyone who has ever used a video or tape recorder. The square button is Stop, the round button is Record, and the arrow-shaped button is play. There is also a button to delete a recording step, or indeed the entire recording, if you want to start over at any point.
Now you are ready to begin your process recording. Open an image, and using the Image > Image Size menu option, resize the image to the required size, in this case 600 pixels wide.
You'll see that the action you just performed has appeared in the Actions palette, as Image Size.
Next, save your image to a new folder. Don't save it to the folder it started in, or Photoshop will overwrite your original images with the resized ones. If you're planning to put the images on a website you can reduce the JPEG quality a bit to produce smaller file sizes. I find a quality setting of eight is about the best compromise between size and image quality.
You'll see that this action too has been added to the Action palette. You can click on the arrow symbol next to the recorded action to open up all the parameters that have been saved. This is useful for checking for errors in long sequences.
When you've performed these actions, click on the Stop button on the Action palette to stop the recording.