1. Build
To build slapd with back-sql under Unix you need to build and install
iODBC 2.50.3 (later versions should probably work). Then, run 
"configure <options you need> --enable-sql [--with-iodbc-includes=<path>] [--with-iodbc-libs=<path>]",
this should build back-sql-enabled slapd.

Under Win32/MSVC++, I modified the workspace so that back-sql is built into
slapd automatically, since MS odbc32 is included in standard library pack,
and it does no bad even if you don't plan to use it. I also could provide
precompiled executables for those who don't have MSVC later (when back-sql
comes into some stable state).

2. Tune datasources and slapd.conf
Next, you need to define ODBC datasource with data you want to publish
with help of back-sql. Assuming that you have your data in some SQL-compliant
RDBMS, and have installed proper ODBC driver for this RDBMS, this is as simple
as adding a record into odbc.ini (for iODBC), or using ODBC wizard in
Control Panel (for odbc32). Next, you need to add appropriate "database"
record to your slapd.conf. See
sample provided in "back-sql/RDBMS_DEPENDENT/" subdirectory. The only thing
worth noting about this is that "dbname" directive stands for ODBC datasource
name, not the name of your database in RDBMS context.

3. Creating and using back-sql metatables
See SQL scripts and slapd.conf files in sample directory .
Create db/user whatever for test, execute create.sql, create_testdb.sql,
test_data.sql,test_metadata.sql from appropriate directory (use 
"mysql < xxx.sql" for mySQL, Query Analyzer+Open query file for MS SQL,
sqlplus and "@xxx.sql" for Oracle)

4. Testing
To diagnose back-sql, run slapd with debug level TRACE ("slapd -d 5" will go).
Then, use some LDAP client to query corresponding subtree (for test database,
you could for instance search one level from "o=sql,c=RU"). I personally used
saucer, which is included in OpenLDAP package (it builds automatically under
Unix/GNU configure and for MSVC I added appropriate project to workspace).
And also Java LDAP browser-editor (see link somewhere on OpenLDAP site) to
test ADD/DELETE/MODIFY operations on Oracle and MS SQL