Simply imagine that your manuscript has a paragraph heading that reads "This is what I'm writing about here". Then write a brief summary of what will follow below it.
For example, I've got an interest in criminal psychology. I might start a study in my subject area along the following lines:
"Title: Examination of a possible correlation between the presence of autism in individuals and the propensity of such individuals to engage in criminal behaviour.
Abstract: In this paper I draw upon the peer-reviewed research of others to establish that 'internal inhibitors' are of far greater importance in preventing criminal behaviour than 'external inhibitors' are.
I also cite research relating to the difficulties that individuals on the autism spectrum may experience in understanding, and complying with, societal norms.
Using these two reference points, I then seek to examine whether people who are on the autism spectrum are statistically more likely to engage in criminal behaviour than those within a standardised control sample of the population as a whole might be.
To achieve this end, I draw upon the research of both criminal psychologists and behavioural scientists, together with interviews that I have conducted with both academics and representatives of the probation service.
I then present my initial conclusions and go on to examine, through interviews with members of the judiciary and others, their possible consequences for the criminal justice system, before suggesting the ways in which further research might be carried out in this field".
[i.e. the abstract makes it clear what the study is about but leaves all of the detail until later in the paper].