1. Unzip the SDK to a directory with folder names.  Note:  The standard SDK is designed for multiplayer coding only.  When compiled you won't see any monsters and enemies and such in the levels.  Only the professional SDK, which is quite large and requires signature of a non-disclosure agreement has the singleplayer code.

2. If you have Microsoft Visual C++ 5 or better, start it up, and under File->Open Workspace and open mp.dsw . This is the project file for the dll and opening it will make compiling your dlls much easier.  Borland or lcc tutorials will hopefully follow soon.  Until then I would strongly recommend grabbing a copy of MSVC5 or 6 off of ebay, where I've seen it for as little as $10.

3. Create a temp directory off of your half-life directory.

4. Create a temp\dlls directory of your half-life directory.

5. Create a text file named liblist.gam in your temp dir with these contents:

// HL Tutorial thingy
// thanks to Beelzabub (beelezabub@gamedesign.net) for assistance on the meanings of this
// also thanks to Ed Resnick (eresnick@U.Arizona.EDU) for more of the same
// still more thanks to Jesper Thiesen (JET@simcorp.dk) for more assistance
// and yes, thanks to nemesis (cctf@captured.com)

game "Wavelength Tutorial" // Game title
url_info "www.planethalflife.com/wavelength/coding" // Info URL
url_dl "" // Downloading URL
version "0.01" // version of mod
size "4" // size of mod
svonly "0" // server side only mod? (1=yes 0=no)
cldll "0" // client side dll? (1=yes 0=no)
hlversion "1009" // version of halflife
type "multiplayer_only" // type of mod--don't know values that go here..
nomodels "1" // game selects player model (1=yes 0=no)
mpentity "info_tfdetect" // name of multiplayer spawn entity
gamedll "dlls\mp.dll" // file and location of dll

6. At this point make whatever code changes you like.

7. Build mp.dll and move it to your temp\dlls directory.

8. Run hl.exe +game temp and you're set. Hooray!