Well, it's not hard actually, for me at least, but i used to compile lots of stuff like miners, wallets, etc. You just need to clone the github repo, ensure that all required package are installed and compile it. Usually, all repos have Readme.md file with requirements and instructions.
In case of solaris wallet, (assuming you have all packages installed) you need to run
git clone https://github.com/solariscoin/solariscoin.git
cd solariscoin
qmake-qt5
make -j 5 - number of CPU cores +1, in my case, core i5 with 4 cores, does not affect the result but greatly speeds up the compile
After some time, if everything goes ok, you'll have solariscoin-qt executable file, which is a solaris wallet qt application.
But, as i said, their wallet is probably outdated on github, it is v 0.4.4 BETA (as said in README), it relates to miniupnpc v1.6, and fails to compile with newer version of miniupnpc, but even if you compile it, it doesn't connect to other nodes.