Fix in basement, how to go about plastering this?

[Fixarjon]

going to build a simpler door and put it into a cut-out hole. My idea was to build a frame of 45*120 and then seal/fill the gaps with mortar. Also smooth on the inside, need to chip away the bigger lump.
Or is it easier to smooth without a frame in place?

 

[neo11]

Cut away the clump and install the frame before plastering.
Then place a 22*95 batten against the white wall in line with the frame, and plaster.