A wide range of tutorials can be found on Worth1000: photography.worth1000.com/
Many standard procedures eg Introduction to Layer Masks (and some advanced) – http://www.gimp.org/tutorials/
Useful instructions for various manipulations including “Straighten a crooked image with Gimp”, “Layer Modes Explained – How To Use Gimp For Beginners” and http://www.tutorialized.com/
Entering “GIMP tutorials” into your favourite search engine will find you lots of examples but don’t forget there is zero quality control on web pages. Just because you find it on the web does not mean it is the best/only/correct way of doing things. By all means try the tutorial then, once you have some experience of the software, come and ask one of the club members who use GIMP (there are a few of us and we regularly win prizes) to see if there are any better ways of achieving what you want.
Other Sources of Information
Cambridge in Colour (http://www.cambridgeincolour.
Image & Digital Imaging Know-How Clive R. Haynes FRPS In teaching and running courses in Photography and Photoshop Clive has produced many notes for students. These are available for anyone who may find the information useful. www.crhfoto.co.uk/knowhowlist.
Online Depth of Field Calculator www.dofmaster.com/dofjs.html
Some useful online hyperfocus and depth of field gadgets johnhendry.com/gadget/index.
A wonderful site from famed photographer, Chase Jarvis. Great assortment of classes, free if you attend live online. http://www.creativelive.com/