After cloning, include dependencies:
git submodule init
git submodule update --recursive --remote
This allows significant performance improvements using Pari/GP (using polredabs); the code will still run if you do not have Pari/GP installed.
After that, run magma in this directory attaching the spec:
AttachSpec("Code/spec");
The following gives a basic test:
sigma := [Sym(4) | (1,2,3,4), (1,3,4,2), (1,3,4)];
// SetVerbose("Shimura", true); (uncomment for verbose output)
X, phi := BelyiMap(sigma);