In recent years, with the increase in use of soft plastics, the humble pirk has been forgotten somewhat. But combining a 100g pirk with a string of hokkais (or simple white feathers) remains an excellent, cheap way to catch cod and mackerel from the kayak during spring and into summer. Of course, other species like coalfish and pollack will also have a go at either the pirk or the hokkais. Hokkais seem to have replaced traditional feathers, but they are almost as cheap as a string of feathers and can be found in most sea fishing tackle shops. I favour the smaller size, though be careful you don't buy a string of sabikis, which are intended just for sandeel or launce.
Pirks vary more, both in size and shape. While there are various new designs, such as Shimano butterfly jigs, most of these are expensive and not necessary from a kayak. Your local tackle shop will probably sell cheap 100g pirks that will work just fine - 100g for me is about right to get the hokkais down in 60ft plus depths. If you're fishing deeper or in big tides, or with heavy mono, you might need more weight to get down to the right depths effectively.
There are various ideas as to why the combination of pirk and hokkais seems more successful than using either on their own. My favourite theory from a friend of mine is that the fish see the pirk as another fish in competition for the smaller but more nutritious sandeel. Occasionally you do hook cod on the pirk itself, but it's as an attractor to the hokkais that it seems to bring you the most fish. In the summer of course, the hokkais are like magnets for mackerel.
Cheap pirks generally come with a set of trebles attached to the bottom. You can fish them like this, but you'll probably hit a lot of snags unless you reel up several metres above the bottom. Even then, you can snag up as soon as you hit the bottom, or you sometimes drift up over a reef and can get caught out by the sudden change in depth.
A better way to rig pirks is to remove the treble and use an assist hook attached to the top of the pirk:
quite pricey themselves. You can use singles or doubles, I've always found one to be enough and it minimises your tackle losses. By rigging the assist hook to the top of the pirk, the bottom of the pirk can strike the sea floor without snagging.
The picture below shows a typical pirk / hokkais rig:
Mr Fish of Jersey has done a nice video of how to tie an albright knot for attaching mono to braid:
For the mono trace to your hokkais you can use a swivel (they generally come with them, but some like Mustard hokkais don't). I like to have my pirk and hokkais rig ready to tie on in my box to save time, and so I wrap the hokkai trace around the top and bottom eyes of the pirk: