Reasonably easy.
The small brass screw in posts that support the motherboard will need placing in the correct holes as the hole positions on motherboards arent standardised.
Where the part of the motherboard that has al the usb, monitor and mouse connections sticks through the case, you need to remove the plate from the original case as they are motherboard specific.
The plate just clips into the new case but again it can be a bit fiddly to get out of the original and into the new case.
There are some springy clips that push againt the motherboard as you postion it and sometimes you have to hold it quite firmly in place while you get the first screw through the motherboard and into the brass post to fasten it in place.
Once the motherboard is fastened in place its just a case of connecting the wires, you have already changed a PSU so you know how easy that part is.
Sometimes the wiring for the buttons and the front of case connections, if the new case has them, can take a bit of working out but usually they are well marked as to which wire is which and as long as you have the page of the motherboard manual handy to double check which wire goes where then you shouldnt have a problem