I believe there will be many who read this article who do not actually know what verisimilitude means, never mind how to go about creating verisimilitude in fiction writing. It is probably best to start off therefore by defining verisimilitude as the state of appearing authentic, likely or believable.
When we are writing fiction, it is all too easy to lose sight of the fact that although our reader does want to be entertained and escape from reality for a time when they read our work, they most often also want to believe for a time that the events we are describing are taking place or are at least capable of taking place. Perhaps other than when we are writing such as extreme fantasy or horror, we therefore have to incorporate some level of credibility in the tale that we relate. There are several ways in which we can do this.
Before we start writing our piece of fiction, we have to ensure that we have all the information we will require to hand. If, for example, we are writing about a particular place in part of our story, we have to ensure that we know enough about the place to describe it to our readers in a way which allows them to imagine they are actually there. It is of course possible that we will be describing a place in this way with which we are already very familiar and that is all good and well, but if this is not the case, we may be required to do some form of research. We may wish to Google the place, look it up in a book, or even speak to someone whom we know that knows it very well. It is advisable to plan our story so that we aware before we even begin of the research we will be required to do and do not find ourselves having to interrupt the flow of our creative powers in order to do said research when it is least convenient.
It is also desirable that the characters who appear in our fiction are believable ones to which people can relate. This also involves forward planning in the sense that we have to build them an identity. Take a sheet of paper for each of your principal characters and simply describe them as though you were essentially writing their CV. Include their name, age, physical description, likes, dislikes, employment status, hobbies and so on, until you have essentially built a profile in each case. Refer to these profiles as you write to avoid contradiction and add notes to them as each character develops within the context of the plot.
When we describe events in our fiction, we have to do so in a way which our readers will see as plausible. Excitement and tension are of course desirable but do not attempt to achieve these effects by making outrageous claims, having characters perform seemingly impossible feats, or by snatching seemingly impossible happy endings from the jaws of certain disaster.
Creating verisimilitude in fiction writing is about credibility and although we always have to warp reality and fact to a certain extent in fiction, we do not wish to do so to an extent where it has lost all semblance of same.