In order to use auto-flow in InDesign, you must first paste the text into the text box on the first page and then flow the remaining text to the other pages. There are a couple of variations on doing that, based on modifier keys.

If you have your pages already created and you have a text box on the master page in which to flow your text, you should use the semi-auto flow method. First click the red plus mark at the bottom-right corner of the text box on the first page to load the overflow into your cursor. Navigate to the next page and hold down the Alt key (windows) or Option key (mac) and click in the text box on that page to flow a page full of text into that frame. By holding down the Alt/Option key, the remaining overflow text will stay loaded into your cursor. Simply navigate to the next page and repeat the process until the cursor returns to a normal pointer, signifying that the end of the text galley has been reached.

Using existing text frames from a master page can sometimes be a little tricky. It may be necessary to override the master in order to use the text frame. Typically, items from the master page are not selectable from the document pages themselves. Holding down Shift/Control (windows) or Shift/Command (mac) while clicking on a master text frame on a document page will override that item so that you can insert text into it. If you also move or modify the size of that text frame, it will no longer comply with any edits to the master page text frame. Use the Enter key (number keypad) to insert page/column breaks into your text rather than adjusting the depth of the text box in order to maintain a link to the master page text frame dimensions.

On the other hand, if you have not created your additional pages but you have set up your margins and columns under the layout menu, you can use the auto-flow method to add new pages to the document until the text has been fitted into the document. After you've copied your text into the first page and clicked the red plus mark to load your cursor, navigate to the next page or insert one if necessary, then hold down the Shift key while clicking in the text area on that page. That will create a text box filling the margins with columns as specified and insert additional pages with the same format as needed.

Placing your text from a text file will load your cursor for you, eliminating the need to overflow the first page. Use the modifier keys as mentioned above when placing text into your first page to initiate auto- or semi-auto-flow.