The dead can hear you speak and receive your mental thoughts directed towards them - directed, meaning nothing more than your intention that they hear you, or see what you are thinking. Invite your person in and give them permission to interact with you.
Your dead loved ones do not wish to cause you any pain or scare you. Sometimes when they reach out, the sense of them might trigger grief or sadness; be aware of this. Tell them that this is okay and to keep reaching out to you and you will deal with the grief and learn how to separate it from their contact with you.
When you talk to them and think of them, try to keep it as happy as possible. If you used to laugh and joke about things, continue doing so. Act as if they are still with you by "pretending" that they are. If they used to drink coffee in the morning with you, pour them a cup and set it out for them. If you see something in a store or online you think they would like, tell them that you're sending it to them. You don't have to buy it; you simply have to intend to send them that gift.
It may be hard at first, but revisit happy memories you have with your person. Imagine they are with you as you do various things - like driving somewhere, imagine they are in the car with you, or sitting on the porch, or going on walks, or going shopping. Talk to them as you normally would, make it as fun and enjoyable an experience as you can.
If you are able, take some quiet time as often as you feel comfortable and visualize you and your person together doing something enjoyable together, in whatever location or surrounded by whatever scenery you desire. In your mind our out loud, repeat a positive narrative to yourself over and over, something along the lines of this: "I know you are with me. I know you still live. I can see you, hear you, feel you, touch you. We are happy. We can communicate and interact easily, and it is fun and enjoyable." It may feel like you are lying to yourself, but what you are actually doing is reprogramming yourself, at the subconscious level, into a psychological state that is more suitable to actually having these experiences.
These habits build a psychological atmosphere that makes it easier for the dead to communicate and interact with you. This communication may come in many forms - signs, synchronicities, physical and psychological sensations, thoughts, mental imagery, ideas, dreams, sudden memories, smells, visual images, sounds, altered consciousness experiences, etc.
Signs and synchronicities usually come in the from of things that have particular meaning to the person or your relationship with them. If the dead person was into music, they will often arrange for you to hear specific songs at meaningful times. If they watched a lot of TV and movies, they might arrange events and nudge you to watch a certain show that will have a meaningful message for you. If you get what you think might be a sign but are not sure, just accept it as a sign and thank your person for it. This will encourage them when they see that you are not allowing doubt to undermine their efforts to show you signs.
Remember, the dead are just people. Their personalities are intact. Show them some appreciation, interest and attention and don't leave it entirely on them to do all the work. Write them a letter now and then, and just intend that they receive the letter. Sometimes they will start writing through you when you write; you might feel them urging you to write things that are obviously from them to you; go with it and let them.
You can invite your person to come into your dreams. Make a deal with them that if anything upsetting or weird happens in the dream, you will understand that sometimes your own fears, insecurities, or other subconscious activities can interfere and affect what happens in dreams. If something like that happens, brush off the weirdness and thank them for coming into your dream and, if you got to see or hear them, enjoy that aspect of the dream and appreciate it. Throw away any weird or upsetting things and don't let them bother you.
You might also ask for a specific sign, something that might have special significance. I know a person whose special sign was a yellow rubber duck. Yellow rubber ducks kept showing up in the oddest places, and her friends kept finding yellow ducks in weird locations and felt compelled to give them to her. Another person asks for peacocks. One asked for a pink bear and found a pink wooden bear on a walk while talking with their dead person. You might see these signs in a TV show, movie, on a billboard, in some random store you visit, etc.
All of this, over time, does two important things: first, it creates a subconscious pattern and an ongoing psychological atmosphere that is conducive to visitation, interaction and communication by making you both comfortable and "in tune" with each other and helps you to better feel and see what may at first be very subtle new sensations and experiences. It sets up the patterns, permissions and expectations going forward that the dead can work with as they learn how to get your attention and interact with you.
Second, these habits form new synaptic patterns in your brain that rewire how you feel and react. You are reprogramming your subconscious into the sense of being back in an ongoing relationship with your person and this will, over time, become satisfying and enjoyable.
Interacting and communicating with the dead in these and other ways have been and are considered a normal part of life in many past and current cultures. It is really only in western, modern cultures that this is considered strange, but there is an increasing amount of people who do this and advocate for it, including a growing grief therapy called "Continuing Bonds" that considers this normal and healthy.