I have some human ones (from College days - I'm a Podiatrist for my sins and at work we use Single Use Pre Sterilised Instruments)
The do belong to me (no theft 
) and have been sterilised but now kept for the Hogs.
Important thing is make sure they are sharp and strong - you don't want to crush the nails and if you do accidently catch the quick a clean cut isn't as traumatic .
I've never made a guinea claw bleed but there's always a first.
Maybe do back paws one night, front ones the next?
The inside claws on the front feet I find tend to turn inwards and curl (especially in GP1)
DD holds the boys with their back against her tummy (like they are sitting in a chair) and one hand under their rump so the weight doesn't go on their 'tail' , they don't have very flexible spines.
I hold the paw firmly, don't give them an option to strop but if they get upset , leave it a few seconds. It's not worth upsetting them but it's got to be done.
Best of all, DD gets her fingers chewed this way
How to make them sit nicely is another matter (most of my human patients attend voluntarily unless there are consent issues. In fact they wait weeks to see me. See Pigs, how lucky you are? No appointment needed)
We have now 4 black paws but one claw is honey coloured.
Four pink paws but they are Rex feet (huge)
GP2 had black paws/black claws and judgey as hell.