There are two things to be aware of. One is your rectus transverse abdominus, which the best way to think of it is as a corset around your stomach.. that's how the muscle works.
The other is tilting your pelvis.
Your TA muscle gets activated by clenching your corset and bracing your abs--act as if someone is about to punch you, you'd tighten and contract them, not push them out, right? So get in a deadbug position and forget about the pelvic tilt for now, and act like someone is stepping on your stomach, clench your stomach! Now think about how your lower back isn't touching the floor. Try to get your lower back to touch the floor, and to remain touching it. This requires a pelvic tilt, and if you can touch the floor, then you did it. The act of keeping your back on the floor is the key to a deadbug. I noticed it is much easier to do with your knees at a bent angle, to activate your pelvic tilt. Another way is to practice on a stability ball. Roll the ball forward and back using only your butt/abs. The forward position is a pelvic tilt.
Another way is to act like you are trying to defecate (don't actually do this!). The muscles used to have a bowel movement kind of force you into a pelvic tilt position, which is what you're doing on the toilet. So remember to brace your abs as you try to tilt your pelvis. Practice with a bent knee position and practice using your abs and pelvis with leaving your back on the floor to bring your legs up (leg raise)
Try looking in a mirror while using the ball to practice the movement. It takes lots of practice before you can do it on command; it's really a mind/muscle awareness connection. Try bracing your abs and tilting your pelvis during an "exhale" of breath, this will help ensure you're only moving the right muscles, not your diaphragm or something. Keep in mind, if these are weak muscles you may have to start out just lying down and practicing these before your muscles can tighten well.