1. Learn to program. Assembly is good to know as part of this as well as C.
2. Get the hardware and devices RKMs if you can.
3. There's a few drivers on aminet and funet to get ideas from, see what the code looks like, etc.
4. Get the OS4 SDK, and some things may require additional seperate documentation for certain things such as the P96 API docs and kit.
5. Learn patience.
6. Find lots of time. It'll take a while to get a feel for things, and it may be a long time before there's any noticable things finally working. It may be very frustrating for a while. For example, it was a long time and a lot of work before Joshua got the first image to display on a Radeon. Would have come sooner if we'd had an AmigaOne, but we were workigin on Classic+Prometheus, and had to get the BIOS POST stuff working before we could really do any display work, when the AmigaOne Uboot x86 emulation would do most of that for us. Took a while to get the memory controller, PLL, and other things working enough to allow us to do other things, and the driver crashed the machine before we figured out the last few details of parsing the ROM. Is that crash caused by a bug in the "display driver" code, or is it because the memory controller configuration has a bit wrong somewhere and will crash no matter what you do in the display code? If the hardware was initted slightly wrong, that can be really hard to find and fix as you may not have a chance to peek registers before a crash and hardware reset to default or blank values. Be prepared to fumble with vague or blatantly wrong documentation if you have that, sometimes poorly commented Linux code (some of that has improved considerably with time though)
7. If you have lots of money, get a JTAG debug kit. Pretty darn expensive, but can be useful for tracking down weird stuff that may be very hard to track down with software debugging inside the OS4 environment. Won't be very common though due to the expense.
If you try to get docs from the chip companies, prepare to nag them for a while. Politely of course, but don't take initial lack of interest as a solid no, unless it's NVidia, they probably won't even call you back to say no. Even after you get your foot in the door, you may still have to nag. I'm waiting for NDA papers for something I was promised them for a month or two ago, I need to call and remind them but I keep forgetting until it's late and everything's closed. And try not to say "Amiga". Talk about writing drivers for an embedded OS or something like that and avoid naming names if you can. Considering some of Hyperions supposed plans, it's not an outright lie to generically talk of some nameless embedded platform... :-D