It is quite simple. If you have a more-or-less standard installation and libosip2 installed at a standard location, it should be sufficient to do:
./configure make make install
This will install siproxd into /usr/local/. If you wish to install it into another location, specify --prefix=<myprefix> when running ./configure. If you have installed libosip2 in an non-standard location use --with-libosip-prefix=<libosipprefix> to tell configure where to find libosip2 (e.g. --with-libosip-prefix=$HOME/lib).
Common features for ./configure:
--enable-static build statically linked executable --with-libosip-prefix=DIR use libosip2 from DIR/include and DIR/lib --with-extra-includes=DIR adds non standard include paths --with-extra-libs=DIR adds non standard library paths
Edit /usr/etc/siproxd.conf according to your situation, at least configure if_inbound and if_outbound. They must represent the interface names (e.g. on Linux: ppp0, eth1) for the inbound and outbound interfaces.
Edit /usr/etc/siproxd_passwd.cfg if you enable client authentication.