Now that all is compiled, you have to copy the built files to an appropriate place (usually in one of the sub-directories of /usr/local).
make can usually perform this task. A special target is the target install. So, using make install allows to install the required files.
Usually, the procedure is described in the INSTALL or README file. But sometimes, the developer has forgotten to provide one. In that case, you must install everything by yourself.
Copy then:
The executable files (programs) in the /usr/local/bin directory
The libraries (lib*.so files) in the /usr/local/lib directory
The headers (*.h files) in the /usr/local/include directory (be careful not to delete the originals).
The data files usually go in /usr/local/share. If you do not know the installation procedure, you can try to run the programs without copying the data files, and to put them at the right place when it asks them to you (in an error message like Cannot open /usr/local/share/glloq/data.db for example).
The documentation is a little bit different:
The man files are usually put in one of the sub-directories of /usr/local/man. Usually, these files are in troff (or groff) format, and their extension is a figure. Their name is the name of a command (for instance, echo.1). If the figure is n, copy the file in /usr/local/man/man<n>.
The info files are put in the directory /usr/info or /usr/local/info
You are finished! Congratulations! You now are ready to compile an entire operating system!
If you just have installed free software, GNU tar for instance, and if, when you execute it, another program is started or it does not work like it did when you tested it directly from the src directory, it is a PATH problem, which finds the programs in a directory before the one where you have installed the new software. Check by executing type -a <program>.
The solution is to put the installation directory higher in the PATH and/or to delete/rename the files that are executed whereas they were not asked to, and/or rename your new programs (into gtar in this example) so that there is no more confusion.
You can also make an alias if the shell allows it (for instance, say that tar means /usr/local/bin/gtar).