How are descent with modification and natural selection related?
They are similar in the way that both theories talk about only certain characteristics being passed down, either acquired over a lifetime or acquired due to variation in alleles.
Assuming you are talking about Lamarck's Theory of Inheritance of Acquired Characteristics, it is stated that whatever modifications or "changes an organism goes through during its life in order to adapt to its environment, those changed are passed on to its offspring". for example, in a very common scenario, a giraffes neck may have over time has grown to reach certain plants. These giraffes will then pass on their characteristics of having a long neck to their offspring. The offspring will then have a long neck due to their parent having acquired it over time.
On the other hand, natural selection is organisms having mutations in their phenotype which will alter their traits as they interact with the environment. Certain traits will mean some organisms are better adapted to their environment and are able to survive longer. These organisms will then be able to reproduce and pass their genes onto their offspring.