Installing stair treads (especially prefinished stair treads) you should measure sorely all details of your staircase. For this purpose you need the stair measuring tool. There are lots of articles and videos about this wonderful tool in Internet.
Also you need to attach the stair nosing to each stair tread and it is mean, that you need to install riser first.
Nobody would recommend you measuring, cutting and gluing each detail of the staircase separately. You need to prepare stair riser, tread and nosing, try to fit all of them and if you made everything well you can start your prefinished stair tread, nosing and riser installation.
In this video you’ll see step-by-step instructions about prefinished stair tread installation with stair nosing and riser. Enjoy watching!