It is cmake based, so typically you'll download the source (either clone the repo or download and extract the latest release tarball), then go into the source, make a build directory, then run
cmake .. (include the two dots in the command). There's extra options you can pass cmake, but that's a generic start. Once that's done, then you can just run make and make install, however, that won't give you a Slackware package, which means it could be more difficult to manage and/or uninstall.
Code:
wget https://github.com/fcitx/fcitx-rime/archive/0.3.1/fcitx-rime-0.3.1.tar.gz
tar xvf fcitx-rime-0.3.1.tar.gz
cd fcitx-rime
mkdir build
cd build
cmake ..
make
make install
If you are semi-familiar with bash, you could probably take the
SBo cmake template and adjust it to this program. This would create a Slackware package you could then install using installpkg. You could then submit this to SBo and become the maintainer of the package so others could benefit from your work
However, looking at their readme shows that they have a dependency of
librime, which is also not available on SBo. That uses just regular make and the install instructions are listed in the readme. If you were to make a SlackBuild for fcitx-rime and want to submit it to SBo, you'd also need to make one for librime, as SBo requires all dependencies to be available on SBo. librime has several dependencies that are available on SBo: leveldb, boost, glog, opencc, and yaml-cpp. But it also requires
marisa, which is not available on SBo.
Overall, it seems it will be quite the project, but certainly not impossible. You'd need to manually build marisa, librime, and fcitx-rime, but you could install leveldb, boost, glog, opencc, and yaml-cpp from SBo using whatever tool you prefer. If you plan on pursuing this, if possible, I'd highly recommend trying to create SlackBuilds for those 3 programs. This would help you immensely any time you need to reinstall these packages (new/different computer, wipe and reinstall, upgrade to the next version of Slackware, etc), plus it would be a great thing to provide to the community so others can benefit from your work as you've undoubtedly already benefited from others work