Honoring and working with the ancestors is an important part of my path. Many spirits are helpful and worth cultivating relationships with, but our ancestors have the unique perspective of having once lived human lives.
Beginning a relationship with the ancestors isn't very difficult. Creating a small ancestor shrine is a good first step. It can include a bowl and plate for small offerings, general representations of your heritage, and pictures or items that belonged to specific family members who have passed on. Our ancestors include not only direct blood relatives, but can also include the blood of those families we have married or been adopted into, one way or another. In addition, our ancestors can also include important figures from the cultures and countries we live in.
Offerings to the ancestors can vary from tradition to tradition. In ancient Egypt, common offerings included bread and beer, and cool water. Apples are another food usually given to the dead. A little bit from a family meal is also welcome. Offerings can be left as long as you like - so long as they don't begin to spoil. What to do with the offerings after again varies with tradition, but commonly they will be put outside (sometimes buried in a very small and shallow hole). If you wish to make an offering to a specific ancestor, and you know what they enjoyed in life, that should be given.
Often I'm asked how communing with ancestors works if I also believe in reincarnation. I do believe that humans can reincarnate after death, but I don't believe that is happens immediately, nor do I view it as a requirement. Some will choose it quickly, others after a long while, some may never reincarnate. So, given that, it's likely that there are some in the unseen world at any point in time. When speaking of specific people, it is possible they have moved on, or perhaps just don't want to answer. There is also another theory... That a bit of our soul is always in the unseen world, and so even if someone had reincarnated, that bit of them can still give aid - and yes, that it is even possible for us to aid yourself in such a way.
Any way you wish to view it, it can still be helpful for us to make offerings in memory of our ancestors, and to honor those who were before us - after all, without them we wouldn't be here.