Altium Designer: ways to split schematic into several sheets
For flat designs I follow these steps:


Create a schematic on your project.
Create a sheet symbol, name it corresponding to the functional block, ie; Power Distribution (Right Click -> Place -> Sheet Symbol)
From the sheet symbol you must create a sheet, (Sheet Symbol Actions -> Create Sheet from Sheet Symbol). This now becomes your functional block schematic.


Its also important to correctly set your Net Identifier Scope to Global. This essentially connects nets and ports corresponding to the same signal together across sheets. This can be done in Project -> Project Options -> Options and selecting the option under Net Identifier Scope (Netlabels and ports global). 

Finally, make sure that all signals are properly connected when you move to layout mode across sheets.