Nothing particularly difficult about a straight flight, just cut the bottom strings square with the bottom of the first riser, lean the staircase up against the top trimmer and make sure the top nosing is level with the landing finished floor level.

Newels have to be fitted but without knowing your exact layout it's difficult to give further advice.

Is the staircase against a wall on one side (or 2 sides) or is it 'freestanding' in the middle of the room?

Is the first tread plain or bullnose or double bullnose etc , so where does the bottom newel fix?

Regardless of these things, it is well within the scope of a COMPETANT diy'er to fit a straight staircase.  It must be fixed well though (to an adjacent wall?) by using Fischer fixings (or similar) through the string on the underside of the string (therefore fixings out of sight).

Newel mortice and tenons have to be glued and dowelled, bit of a struggle to get things chopped into place sometimes, depending how much room you have to play with.

So the handrail is fitted to one of the walls then?

No newels?

It just gets easier