Main references are here and here.

1. Basically, need to have C library and compiler capable of multilib. I use getslackpack to download required packages from Eric's multilib site. Install as instructed.
2. I already have a 32-bit Slackware tree downloaded using getslack which I use for my 32-bit chroot installation. I use massconvert32.sh script on this tree. The massconvert32.sh script can be used to update as well (built packages are not rebuilt). Install as instructed.
3. My slackpatch script has been updated to handle 'blacklisted' 64-bit versions and 'upgraded' compat32 packages

Update20180903

Read here. I now have a more specific script to get multilib stuff (previously part of getslackpack script),

• use getslack-multilib to download alien_bob's multilib stuff
  • compat32-tools glibc(&friends) gcc(&friends)
  • compat32 library packages
• upgrade pure-64 glibc/gcc packages counterparts
  • upgradepkg –reinstall –install-new *.t?z
• install compat32-tools
  • some useful helper scripts
• install 32-bit layer support libraries
  • upgradepkg –install-new slackware64-compat32/*-compat32/*.t?z
  • obviously, can be used to upgrade as well