The hard part is (probably) the transition from Prokaryotes to Eukaryotes. The transition from unicelular Eukaryotes to multicelular Eukaryotes is (probably) easy [1] .

Edit: [1] From https://en.wikipedia.org/wiki/Multicellular_organism#Occurre...

Multicellularity has evolved independently at least 46 times in eukaryotes, and also in some prokaryotes, [...] . However, complex multicellular organisms evolved only in six eukaryotic groups [...]