Hello,

I am planning on installing 1/2" engineered hardwood on stairs with planks and stair nose. I already removed the carpet, removed nails/staples, cut existing noses and sanded each step.

I want to put in poplar risers and my question is does the riser go on top of the planks or do I butt in the planks against each riser ?

If the latter, would the cut need to be perfect in order for gaps not to appear?

You can install all of the steps first, then install the risers. You may need to plane the edge of the riser that sits on the flooring, leaving a small gap at the top which will be covered with scotia/coving. The nosing should project to cover the thickness of the riser and scotia, one and a quarter to one and a half inch. I use quarter inch plywood for the riser so this technique works well. There is no need for a ¾ inch thick riser.
If you use a ¾ inch riser, you will need to install the riser before the nosing. Any gap between the flooring and the riser will not be covered with the scotia.

I find it is easier to work from the bottom of the staircase to the top doing riser then step, riser then step, etc. As you said you won't need a perfect cut on the back of the step as the riser will cover it and also if you need to adjust the step to get the nosing overhang just right this method will let you do that as the riser will hide any gap at the back of the step. Also, be sure to check the stair building code for rise and run specs....the landings' rises, top and bottom, also should be code compliant.
With the riser resting on the tread you should be able to get it 'perfect' as you can sit it on the tread and cut the top edge square with the subtread of the step above....this may negate the need for scotia under the tread nosing unless it's there for aesthetics.

Thanks for the replies.
One more question, if my risers have tiny gap on the side (between riser and skirtboard), can i just use stainable wood filler ?

Yes.

Thanks. I was worried this might crack over time due to stress on the stairs.

Some wood fillers shrink as they dry and some don't. Use one that won't shrink.