Lay a blanket on the floor and place your baby on their tummy while they are awake and alert.
Get down at their eye level — face to face works better than anything else — and put one bright toy just beyond their hands.
Talk to them the whole time. Stop the moment fussing starts rather than pushing for a target duration. Several short sessions beat one long one.
Time on the tummy builds the neck, shoulder and back strength that rolling, sitting and crawling all depend on. Placing something just out of reach converts passive positioning into an active push, which is what develops the muscle rather than merely tolerating the position.