what tips could you give, including tools, materials etc.
Plenty of putties to play around with, but for two beginners suggestions, get either some fresh greenstuff (check if sellers keep it chilled) and mix it with a bit more yellow, like
PSI now recommends. Or procreate.
For tools, I'm fond of wax carvers. You can get cheap sets of multiple tools, though while they can be useful for finding out the precise shape you prefer, you'll likely end up using only one or two for most of your work because of that, and sometimes the quality can leave something to be desired. IMO the most useful types are variations on a theme anyway: a relatively sharp blade with a slight point and wide surfaces on the sides, and a small, slightly convex 'fingertip' with a blunter edge. You can usually find dental-quality versions on ebay (or digging deeper for some dental retailers) for not a lot of money - about £2.50-£4 each - and the different types of carver have distinctive names you can search for. Try looking for
zahle,
lecron,
vehe or
hylin carvers, for starters. (Zahle wax carvers are the 'wax #5' carvers that are sometimes mentioned and recommended in more arcane sculpting discussions)
Colour shapers - those silicon-tipped 'paintbrushes' - are useful too. The extra-firm black variety, called clay shapers, are great for putting the final smooth and polish on greenstuff and procreate. (Smoothing putty is like an entire sub-discipline in itself) Size 0 is best for most wee little 28mm sculpts and conversions (size 2 for larger objects and surfaces) though specific shape is more a question of personal preference. Me, I like using a 'round cup' for smoothing. In my experience it smooths without leaving tiny grooves that the square corners of the flat chisel might. These and the other shapes can be used for general sculpting too, but IMO when you're dealing with slowly-hardening greenstuff or procreate, the extra push of a metal tool is hard to beat.
On that note, Heresy miniatures sells clay shapers, including a couple of double-ended types with added wire loops. I've got them both, and while I don't use them often, I think they've got interesting potential. But I'm personally too comfortable with my wax carvers.
And on
that note, there are plenty of other tools and objects that can be used. Tom Meier had a photo online, showing off his: a blunted hobby knife blade; a bent, shaped needle; and a homemade wire loop tool. I also use different shapes of knife blades, needles and other things on occasion, for specific purposes, and I've seen others get great results with shaped, polished toothpicks and other wooden tools. But stop me if I'm starting to confuse you.

When sculpting, you'll want some kind of lubrication to stop the tools sticking too much to the putty. It can be water-based, like... water... or oil-based like vaseline. Lots also swear by their own bodily secretions - spit and forehead grease.
Me, I like avoiding too much muss and cleanup and opt for plain water, although it can cause problems if you wet your tools too much, running over your sculpt or mini and preventing the putty sticking to
anything. You don't want droplets of water hanging off your sculpting tools - just a slight moistening. For that I use a bit of wet sponge (recycled blister sponge is very useful for that!) in a small dish.
Then there's a nice, logical procedure to sculpting. Putty is softest and most sticky when freshly mixed and warmed by your hands, so you'll want to stick it on and push it into rough shapes at that point. Then, as it cures, the increasingly firmer and less sticky stages allow you to better refine the shapes, add details and smooth the surface. The less 'give' there is, the finer you can work without accidental dents, dings, fingerprints, deformation of details, and so on. Up to a point, of course. You can reasonably have a two-hour working window before the putty becomes unworkable, depending on the temperature around you.
And there's another tip. The rate of epoxy putty polymerisation, like other chemical reactions, is dependent on temperature. (Thank you, GCSE chemistry) If you want to speed up the curing process, you can place the sculpt or conversion somewhere warm - an airing cupboard, under a desklamp, or in a toaster oven or homemade 'putty oven'. (Basically a lightbulb fitting in a large tin can. Me, I freecycled a big old combination microwave where the microwave setting was broken but the electric oven element still worked. Useful for putty curing and baking big polymer clay things. But I digress.

) Don't heat it too high, though. Apart from causing disaster if you happen to be converting a plastic mini, it can cause the putty to bubble and deform too.
Conversely, low temperatures slow down curing. The main way to make use of that is if you've mixed too much putty to use in one sitting. Store the excess in the freezer and it'll stay workable for 1-3 days, depending on what point of the working window you froze it. In a similar vein, unmixed greenstuff that you don't use up quickly can be kept fresh in the fridge. (I've heard warnings against keeping it in the freezer, but I'm not sure about that since that's what I do, with no apparent ill effects.) Greenstuff can go off, and that causes problems with mixing, workability and curing.
What else can I say? I haven't sculpted too many cowls, so the best I can offer you is to make sure you've got your reference photos and materials handy. Then stick a bit more putty than you'll think you need, onto the head of the mini (a wee bit easier to nick off excess than smooth in extra) then form the rough, overall shape. (A vague three-sided pyramid almost enclosing the head, open at the face side, if you mean something like a loose hood rather than Batman's cowl!) Then form the loops of hanging cloth under the jaw, and the drooping cloth over the forehead and the blunt point at the back.
Lastly,
these boards, and their sticky topics, might hopefully be useful to you too.