You need to be very specific.
Firstly get hold of the proposed plans. Look at these in the light of the development plan for the area and try and find any areas where they breach local planning guidlelines.
For example:
3.1 The development would not appear to be in accord with Policy H9 Residential Layout and Design, in that it is not compatible with the density provisions, and will impact local amenities. There is inadequate provision for any amenity areas and will make demands on what limited amenity there already is
3.2 The buildings proposed are out of character with the current developments in the area.
3.3. The development will be, at this size, an un-neighbourly intrusion onto the properties surrounding it.
This is from an objection to a development which although granted, was subject to conditions arising from this objection, inasmuch as they had to reduce the height of the roof and create more space around them - i.e go for a smaller floor plan.
I think you get the idea? Be very factual and concentrate on areas where you can see the plan goes outside the guidelines. Also watch the supposed facts presented. I got a planning application rejected once (as a private citizen) because I was able to show that the planing consultant submitting the plans had deliberately mis-stated the area of the site in order to make the room density fit. Once it was pointed out, the planning department checked and found I was right and kicked it out.