There are a variety of ways to make your writing more cohesive, here are a few ideas:

1. conjunctions: these are the words like and and but
2. pronouns: pronouns (it, they, this and that etc) are generally linking words as they link back to nouns
3. vocabulary: another way to link is to use the same or similar word again.

How to do it?

There is no easy answer to this question. However, one very useful piece of advice is to make sure that each sentence starts with a reference back to the previous sentence. Look at this example:

There are many people who claim that global warming is the most significant threat facing us today. They argue this because it is a danger not just to the current generation, but also to the generations to come. Indeed, it is this threat to our future that is of most concern. For instance, some research shows that one effect of global warming might be there will not be enough food to feed the world in the near future. If that did happen…

 

If we look at how this links together, we see:

A common mistake