If you are cleared to descend "via" a published RNAV arrival, then if you want, you can select the final platform height in the MCP.
Having done that the aircraft will attempt to make all the height and speed constraints within the RNAV STAR. You do need to remember to select QNH at the appropriate point. I put that point in the fix page, and that serves as a prompt.
I use the word attempt, because the 744 is a simple beast. You do need to watch it closely.