I highly recommend using the xP installer instead of these directions, however, if you want to do it the hard way...
This is billed against PINE v4.10 by the University of Washington.  (I'm trying to update it for the latest version of PINE (v4.21)).

(Step 1) Download PINE from Sourceforge:
       ftp://xpine.sourceforge.net/pub/xpine/xp-0.0.12/pine4.10.tar.gz

(Step 2) Download XClass from Sourceforge:
       ftp://xpine.sourceforge.net/pub/xpine/xp-0.0.12/xclass.tar.gz

(Step 3) Download xP from Sourceforge:
       ftp://xpine.sourceforge.net/pub/xpine/xp-0.0.12/xp-0.0.12.tar.gz

(Step 4) Explode the PINE tarball in /usr/src
 

E x a m p l e cd /usr/src tar xvfz pine4.10.tar.gz

(Step 5) Explode the XClass tarball in /usr/src/pine4.10 and move the archive directory to a directory under PINE called "xclass":
 

E x a m p l e cd /usr/src/pine4.10 tar xvfz xclass.tar.gz mv xclass-new /usr/src/pine4.10/xclass

(Step 6) Explode the xP tarball in /usr/src/pine4.10 and move the archive's directory contents to a directory under PINE called "X":
 

E x a m p l e cd /usr/src/pine4.10 tar xvfz xp-0.0.12.tar.gz mv xp-0.0.12/* /usr/src/pine4.10/X

(Step 7) Patch the PINE distribution
 

E x a m p l e cd /usr/src/pine4.10 patch pine.c /usr/src/pine4.10/X/pine.c.patch patch mailindx.c /usr/src/pine4.10/X/mailindx.c.patch patch mailview.c /usr/src/pine4.10/X/mailview.c.patch patch makefile.lnx /usr/src/pine4.10/X/makefile.lnx.patch patch headers.h /usr/src/pine4.10/X/headers.h.patch

(Step 8) Move the new files into the PINE directory
 

E x a m p l e mv /usr/src/pine4.10/X/pinexovr.h /usr/src/pine4.10/pine mv /usr/src/pine4.10/X/picoxovr.h /usr/src/pine4.10/pine mv /usr/src/pine4.10/X/mailindex.c /usr/src/pine4.10/pine mv /usr/src/pine4.10/X/bld /usr/src/pine4.10/pine

(Step 9) Build the PINE distribution (Ignore the link errors)
 

E x a m p l e cd /usr/src/pine4.10 build slx

(Step 10) Build the XClass distribution
 

E x a m p l e cd /usr/src/pine4.10/xclass ./configure make

(Step 11) Build the xP distribution (Don't worry about the link error)
 

E x a m p l e cd /usr/src/pine4.10/X make

(Step 12) Link for the xP distribution
 

E x a m p l e cd /usr/src/pine4.10/pine ./bld

(Step 13) RUN xP!
 

E x a m p l e cd /usr/src/pine4.10/pine ./pine