A good way to write the main body (paragraphs 3 and 4) in your task 1 answer is by developing the two ideas that you wrote in the overview (paragraph 2).
To show you what I mean, I've highlighted parts of the answer below.
The diagrams illustrate some proposed changes to the central area of the town of Islip.
It is clear that the principal change to the town will be the construction of a ring road around the centre. Various other developments with regard to shops and housing will accompany the building of this road.
Looking at the map of Islip as it is now, we can see that a main road runs through its centre from east to west. The second map shows the planned pedestrianisation of this road. Traffic will be diverted on to a dual carriageway that will form a ring around the town centre.
Currently there is a row of shops along either side of the main road. However, it appears that the shops along the north side of the new pedestrian street will be demolished to make way for a bus station, shopping centre, car park and new housing area. The shops along the south side of the street will remain, but it seems that the town’s park will be reduced in size so that more new houses can be built within the ring road.
Can you see what I've done? The first sentence of the overview (green) is developed in paragraph 3, and the second sentence of the overview (blue) is described in more detail in paragraph 4.
Tip: Try reading all of the 'green' sentences together, then read the 'blue' sentences separately. This should help you to see how the overview and main paragraphs are connected.