Asuming sql server here:
Ask your dba to restore database backup file "GOSL", "GORT" and "GOSLDW".
These are found in c8\webcontent\samples\databses\sqlserver (or something like that path, don't have a C8 in front of me)
Ask your dba to create an sql user called "samples" (with for simplicity, the same password)
Ask the dba, to make this new "samples" user, db_owner of the 3 restored databases.
Copy c8\webcontent\samples\content\cognos_samples.zip to: c8\deployment.
Open cognos connection, go the data sources, and create a new datasource for each of your restored sql databases. The data source names HAVE TO BE the next format, in order to work.
db GOSL = "GoSales"
db GORT = "GoRetailers"
db GOSLDW = "Go_Data_Warehouse"
Use the "samples" user for logging into it, and make sure to tick the box that saves the password, during the datasource creation. Use the testbutton to doublecheck.
In deployment, import the cognos_samples.zip package.
The samples are now ready to use.
A full step by step is mentioned in file "inst_cr_winux.pdf" on page 163 to 172 located on both the Cognos 8 cdrom and your cognos 8 install directory.