A number of different tree types will produce suitably straight shoots. Dog-woods are good, Hazel, Alder maybe? Just use your judgement, and experiment a bit.
Assembly. (In fact, one ‘prepares’ one’s birch) Your chosen twigs (that doesn’t feel like quite the right description but I can’t think of anything better) all need to be aligned with the thick ends together. And now the fiddly bit, you must rotate them individually until they all curve in towards the middle so that there are no loose bits sticking out, without dropping the entire bundle. An elastic band could prove useful here. That last bit is important, because the stroke of the birch as a whole lands with a springy cushioned (by its own mass) and evenly distributed impact, but any stray ends will whip and slash into intimate or undesirable areas. Take your time with that last bit. Finally, the ‘handle’ is formed by binding the thick ends tightly (and I mean tightly) together. The binding would traditionally be ‘string’, for want of a better word, but in these modern times a spiral of ‘gaffer tape’, or ‘duct tape’, or ‘electricians tape’ functions perfectly well. You are making a hand grip, and so the binding only needs to be long enough for that purpose. However, the longer the binding, the better controlled your stroke will be, and the tighter the business end will be packed together. The handle could be anything up to half of the total length.
The thickness of your implement should be dictated by your particular variety of twig thickness, the specific type of impact effect you want, and the size of your hands. Half a dozen twigs will sting like a multi-stick switch, while 20 or 30 twigs will have what feels like a ‘soft’ impact, with air resistance having a part to play, balanced against the increased weight. The only option here is to experiment, and check the comparative effects on one’s willing ‘victim’. If your finished article could function as a broom, then it should definitely make an effective swisher.
Speaking of which, what should we expect by way of impact? The first thing to bear in mind is that no two birches are ever the same. Add in the variations of twig thickness, twig quantity, bundle compactness, and length, and the impact is almost infinitely variable. But generally speaking, one may well find that one’s recalcitrant subbie is distinctly underwhelmed by the first few strokes, not much more than a little bit of minor stinging from stray ends. And unlike other implements, like paddle or hairbrush, there is no major point of impact. However, by the time 20, and then 30 strokes have been vigorously applied, you will observe some uncomfortable twisting and turning going on. From first-hand experience (I always make it a rule to test implements personally), the first few swishes are just like being whisked, nothing much to report, but a subtle warmth develops. And as the swishing continues, that warmth sneaks up on you until it is hot, and then burning. And those minor little stings that you cheerfully ignored a few moments ago are landing on hotly tenderized skin. Now you will be paying attention!
And now a few words of caution. Never, ever, attempt birching without a fully functional vacuum cleaner on hot standby! Little bits of bark, bud and twig-end fly absolutely everywhere! And while our darling subbie only has the occasional impact on her rump to contend with, the health & safety police would insist that us domly types wear protective goggles, high vis vests, hard hats and all manner of other safety equipment. And subbikins will be picking little woody bits out of the darker recesses of her nether regions for hours afterwards.
General safety. After a dozen or so strokes, the buttocks will be criss-crossed with tiny pink welts, and taking on an overall pale pink hue. Little red spots appear all over the skin, and although these begin as red spots (not skin breaks) where the twig-ends have landed, over-enthusiastic application will, in time, cause the skin to break, in a minor and eventually major way. And while we may indeed threaten that we are ‘going to take the hide off her’, I think we will all agree that this eventuality is less than desirable. I recommend having antiseptic cream on standby, and look at all the fun you can have as ‘that’ takes effect!
Care of your birch. Fresh-cut twigs look after themselves, they just need careful stripping before assembly. But the ‘home decoration’ version are pre-dried, by definition. Used ‘right out of the vase’, they will disintegrate in minutes. Historically, birch rods were stored in a tall jar of water in order to keep them supple. More than that, it was common for the storage jar to contain ‘brine’ (salty water), as this had the fortunate effect of applying antiseptic as well as chastisement ~grin~ (we all like to save effort, don’t we?). What the Swedish like to refer to as ‘astringent benefits’ will surely add a certain ‘je ne sais quoi‘ to any disciplinary session. Enjoy.