-Use Editor/Assembler on the machine itself (still one of the more common approaches)
-Win994A includes a fairly decent assembler - http://www.99er.net/win994a.shtml
-WHTech has other tools including a c99 cross-compiler and assembler: http://ftp.whtech.com/
WHTech is the primary archive - though it's a bit overwhelming. But pretty much all software, hardware docs, etc, are available there. It's undergoing a reorganization at the moment to hopefully make it more useful.
Secondly useful archive is at 99er.net - http://99er.net/download2/
The best hardware tech page would be Thierry Nouspikel's site: http://www.nouspikel.com/ti99/
Emulators I'll just take from Wikipedia:
* Classic99, a Windows-based emulator including TI-99 ROMs under license from Texas Instruments http://www.harmlessl...tware/Classic99
* Win994a Emulator - a Windows-based emulator of the TI-99/4a http://www.99er.net/win994a.shtml
* TI-99/Sim - a software simulation of the TI-99/4A http://www.mrousseau...ograms/ti99sim/
* PC99 - a commercial DOS-based emulator licensed by Texas Instruments to sell ROMs http://pw2.netcom.com/~mjmw/
* MESS - a multiple system emulator that supports the TI-99/4A http://www.mess.org/
* MacV9T9 - a Macintosh based TI-99/4A emulator http://pytheas.club....sh/MacV9T9.html
* Ti994w - a Windows based emulator with 80 column support http://members.ziggo...94w.html#ti994w
As for guides getting up and running, basics of compiling, ie: turning the above into something you can actually use to get started, there's not a lot out there for that. The TI community is still relatively young so far as the modern age of interest goes. I'll post a quick example of using Editor/Assembler under Classic99, see if that helps.