1)
Download JAZZ by Mika Heiskanen & Jan Brittenson and install it in your HP48
or use EMU48 to run JAZZ.
Jazz has documentration and explanation about how to use it.
See also http://www.hut.fi/~mheiskan/ for more information.

2)
If possible buy the book An Introduction to HP48 System RPL and Saturn Assembly Language,
by James Donelli from Da Vinci or AMAZON

3)
Download some of the following
Tutorials from HPCALC.ORG for HP48 and for HP49
Programming in User RPL doc by Eduardo Kalinowski

4)
Use and study the following examples:

• Search for SYS RPL Posts and Examples by John Mayers at comp.sys.hp48
• F.H. Gilbert, from France, wrote one of the best documentation about Machine Language-ML.
  It is higly recomended to the beginner in ML use and study this document.
  Download the FH Gilbert ML Programming Document.

A little more about SysRPL and ML

Area48 Site is a site for HP48/49 beginners and intermediate users.
Maintain a site for advanced programming is out of our purpose.
You can, however, find more about Sys RPL in the folowing sites:

1 - http://www.hut.fi/~mheiskan/, by Mika Heiskanan - Finland
2 - http://www.hpcalc.org, by Eric Rechlin - USA
3- The HP48 Home Page, by Eduardo de Mattos Kalinowski - Brazil