The pattern you are working should have a 'finished' size. My recommendation is to take the finished size measurements and add the minimum 3" on each side to accommodate. (In otherwords if the finished product is 8 x 10 you would cut the fabric to be 11 x 13) ....since this is a multipage pattern (usually an even number of pages - 2 or 4?) you will need to work the project in quadrants or halves, depending on the number of pages involved. Hopefully, this makes some sense. For example is 8 x 10 and it's a 2 page project, the first page will be used on half the fabic (4 x 5) so you would start working the project in the center of the 4x5 piece. Also, for me anyway, I find it helpful to just take the instructions and tape them together to make the patern one page -- once it's all together, you can see where the center of the design is and fold the pattern to work it easily. This particular pattern is apparently designed to work on several different thread counts, so you will need to figure the finished design size based on the thread count of the fabric you are working. In instances like this, many times it is just easier to take the pattern to the craft store and let one of the associates help you figure exactly what you need to get the project underway.
Please let me know if I can assist further.