The right files to kitbash from? You purchase them from WWG. A kitbash is just a modification of their files. Just don't go sharing your modifications with others - it's ok for personal use. If you have something you think would be appreciated by others, you can follow the for a kitbash.
If you're using GIMP, here's the basic process I use kitbash stuff:
1) Start GIMP
2) Open the PDF file you want to modify.
3) GIMP should prompt you for which pages you want to import, and at what resolution. You can select the pages you want - they get imported as layers - and you want to change the resolution from 100ppi to 300ppi (the tlx stuff is all 300ppi, I believe, and many non-tlx sets too?)
4) Now you can modify the images. When you're happy with them, save the images as PNG files. If you intend to submit the kitbash, grab the header image that's linked in the submission guidelines and use it on your images rather than the normal product banner.
5) Use OpenOffice Draw to create a new document. Create as many pages as you have PNG files saved. Insert each PNG file on a new page. Right click on the image, select Original Size, and it *should* perfectly fill the 8.5x11 inch page. If it doesn't, something probably has gone wrong.
6) When you're done, you can export the whole thing as a PDF file, being sure to use some compression, as these things get huge.
That's about it. If you don't plan to submit kitbashes, then you can go wild with how you modify the files and noone else will be the wiser. Just please don't go sharing WWG stuff with other people. If you plan to submit the kitbash, make sure you keep your changes within the L-shaped registration marks, as those define the effective area for automated cutting machines. Also, if you're just re-texturing tiles, try to not alter the location of the tiles on the page, so that way the existing automated cutting machine files will "just work".