Unless you are taking on
the installation of Honjo aluminium mudguards, putting these essential accessories on your bike is one of the most straightforward jobs required to keep you and your bike clean and happy.
SKS, as noted above, are probably the best available. The only tool that may not already be in a basic bicycle tool kit is a pair of bolt cutters -- handy for trimming stays (make sure you file the ends afterwards and put the supplied plastic end-caps on).
Complications may occur if you have a "racing bike" not designed for mudguards. That is, without sufficient bridge/brake clearance and without eyelets to attach stays.
Then you're stuck with those silly "blades" or other excuses for mudguards, or to cutting and rigging fenders around the contrictions.