Hacking your plants into smaller bits and replanting them is highly satisfying, without making anyone cross, or you fat - hurrah!
There are several ways to do it, and as always with gardening, common sense will let you know. The easiest are those (like pulmonaria and lychnis) which form separate plantlets growing off the main plant. All you need do here is to pull them away.
Then there are larger clumps, (viola and alchemilla for example) which will need digging up and pulling apart into a few equal sized pieces - not too small though; each piece needs enough root and shoot on it to flower in the same season.
Next are the ones which are harder to pull apart, (nepeta, rudbeckia (above), aster). For these clumps the advice is to use two garden forks back to back to wrench the roots apart. In practise I find this pretty awkward and usually end up using a combination of my feet and hands, with a bit of chopping thrown in for good measure.
Lastly, for those plants with tough centres, like delphinium, agapanthus and hellebores, there is the highly satisfying spade or knife method, where you chop or slice the clump into smaller bits using a sharp spade or a bread knife.
It’s not open heart surgery, so leave any nerves at the door and enjoy a bit of positive destruction.