Windows flashing instruction for the D-Link DIR-300 to DD-WRT (by Brainslayer, modified by Sash)

configure your local ip to 192.168.20.80
connect your lan cable to the WAN port of the router.
plugoff the power cord and replug it.
now enter the redboot using telnet and ip 192.168.20.81 and port 9000. connect your lan cable to the dir300 wan port.
you might need several tries since its only available for 1 second after aprox. 5 sec. of booting 
(in the appendix is a windows instruction for easy connecting to redboot)

DIR-300 Info:
IP: 192.168.20.81/255.255.255.0, Gateway: 0.0.0.0
Default server: 192.168.20.80

now start a local tftp server on your computer and place ap61.ram as well as ap61.rom in the root dir of this server

back to the redboot enter:
load ap61.ram
go

now a new temporarily bootloader should start. (on serial console it will display some warning. but you dont need to care about)
lug you lan cable to the lan port of the dir-300.
reconfigure your ip to 192.168.1.2 and connect via telnet to 192.168.1.1 and port 9000.

important: while doing the following steps. never plugoff the lan cable or the power cord

fis init
About to initialize [format] FLASH image system - continue (y/n)? y
*** Initialize FLASH Image System
... Erase from 0xbffe0000-0xbfff0000: .
... Program from 0x807f0000-0x80800000 at 0xbffe0000: .

ip_address -h 192.168.1.2
IP: 192.168.1.1/255.255.255.0, Gateway: 0.0.0.0
Default server: 192.168.1.2

load -r -b %{FREEMEMLO} ap61.rom
Using default protocol (TFTP)
Raw file loaded 0x80080000-0x800a8717, assumed entry at 0x80080000

fis create -l 0x30000 -e 0xbfc00000 RedBoot
An image named 'RedBoot' exists - continue (y/n)? y
... Erase from 0xbfc00000-0xbfc30000: ...
... Program from 0x80080000-0x800a8718 at 0xbfc00000: ...
... Erase from 0xbffe0000-0xbfff0000: .
... Program from 0x807f0000-0x80800000 at 0xbffe0000: .

reset

now the unit will reboot and the new bootloader will be executed and is waiting for your connection.
this wil take 30+ sec.

connect to the bootloader with telnet on port 9000 and ip 192.168.1.1

now do the following: (consider that fis create operations are taking sometimes several minutes to finish)
load -r -b 0x80041000 linux.bin 
fis create linux

fconfig boot_script true
fconfig boot_script_timeout 4

now type fconfig again and configure the bootscript to
fis load -l linux
exec

now type reset again. the unit should now boot dd-wrt and work as known from other atheros based platforms

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Appendix:
download putty.exe to c:\
create a file an c:\ called redboot.bat with the following content:

rem echo off
:start
rem ------------------------------------------------
ping 192.168.20.81 -n 1 -w 1 > NULL
IF ERRORLEVEL 1 goto start

rem putty
rem ------------------------------------------------
break
putty.exe telnet://192.168.20.81:9000 -m redboot.txt
exit


create a file on c:\ called redboot.txt with the following content:

^C


start the bat file and power on the router.....here comes redboot