Hmm... I don't think it is too simple, but let's try to get this going...

First of all you need to have a cross-compiler. Do you have an ARM compiler compiled for x86? If not, you need to get one. I'm sure Google will help you on that (quickstart: compile gcc bundle with a crosscompiling switch enabled and choose ARM as the target architecture).

Then you can return to the Trolltech guide and look for qmake spec file as described in the guide. I'm sure all files for ARM are there with Qtopia Core so you needn't change anything, just follow the guide.