Ok...
Manually creating previous and next navigation tabs for gallery attachment pages
1. For a text link, e.g. "Next", the code would be:
<a href="URL_OF_NEXT_ATTACHMENT_PAGE_HERE">Next</a>
For a thumbnail link instead of the text tab, the code would be:
<a href="URL_OF_NEXT_ATTACHMENT_PAGE_HERE"><img src="URL_OF_NEXT_IMAGE_HERE" width="100" /></a>
(The width number is a suggestion - you can change it. The URL of an attachment page is the post URL plus the image name, without the ".jpg" part, folllowed by a slash.)
2. You create these links, then create the following table code and paste it in the description box of the images:
<table width="100%">
<tr>
<td align="left">LEFT_LINK_HERE</td>
<td align="right">RIGHT_LINK_HERE</td>
</tr>
</table>
For the first and last images you can have a link with the appropriate text tab instead of "Previous" or "Next" (e.g. "Back to post"), or you can leave that part of the table code (the part shown in capitals) empty, if you want nothing instead.
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For a simpler alternative you can omit the table - just write this in the description boxes:
<a href="URL_OF_PREVIOUS_ATTACHMENT_PAGE_HERE">Previous</a>
<div> </div>
<a href="URL_OF_NEXT_ATTACHMENT_PAGE_HERE">Next</a>
(This way you get both tabs aligned left, the one below the other. Less work to do; but less elegant.)