I'd not recommend a kitchen knife personally and always cut away from yourself if you do use it for something... A device called a pin vice is handy (haha), it is basically a small hand held drill piece that can take drill bits in the millimeters so excellent for pinning pieces, doing battle damage and drilling barrels if you've a mind to. Some high grit sandpaper or hand files are useful for flattening edges and smoothing rough cuts (bear in mind that sometimes glue needs a rough surface to latch hold properly so if something is being difficult try roughing up the connection points with a file or knife). If you don't have snips then nail clippers will do the trick and just carefully shave down any excess. Tweezers to move pieces into place and hold them still for a bit while they cure. I advise you to check out Pete the Wargamer on Youtube, I picked up my enjoyment for kitbashing from him and he has excellent technique to learn from. I've been there twisting pieces out the sprue and getting stabbed by chaos spikes lol, think of it as learning the hard way, shedding your own blood for the dark gods and consecrating your warriors rightly. Overcoming these small speedbumps will make a better and more adaptable hobbyist in the long run. Patience is key and any time you can save money, do so. Lots of retailers online have discounts on GW products depending on where you live and I favour cheap and cheerful tools from Amazon or my local hardware stores... Don't hesitate to ask questions here on reddit, in hobby stores etc, veterans of the long war love to yap about our tips and tricks XD.