I customise my Filofax contents quite a bit and above is what I use to do it. It is a Fiskars Portable SureCut™ Paper Trimmer 22 cm -A5. I cannot do without it – I use it at least twice a day for trimming all kinds of paper. This is another thing I think Filofax should add to its offerings – a nice light inexpensive paper cutter with the Filofax logo on it. Rather that than another hideous high end designer collaboration.
It is light and the cutting bits are well protected so you can’t hurt yourself while using it. It has holes to fit in a file and it is light and portable. If you blunt the blade through use you can replace it. It is cheap and works like a dream. It has markings for A5, A6 and A7 but the squares mean that you can cut your paper to any size you want. It can’t do huge wodges of papers but can definitely do about five at a time. I paid about £10 for mine.
Fiskars is a Finnish brand but their products are available from hobby and scrapbooking sites and from Fiskars themselves. Also on eBay and Amazon.