Find the move that opens the board.
Good solves often begin with the smallest restriction. Look for a cell or region that has fewer possibilities than its neighbors, then use it to learn something about the rest of the board.
Start where the clue is tightest
A single-cell sum fixes a value immediately. An almost-full sum region can be just as useful: subtract what is already placed and you know what the last cell needs.
Small upper limits are restrictive too. A two-cell <2 region allows only 0 + 0, 0 + 1 or 1 + 0. Compare those possibilities to the actual dominoes in the tray.
If a region needs 9 and already contains 4 and 3, its last cell needs 2. Look at both halves of every remaining domino containing 2 before choosing one.
Treat each domino as a promise in two directions
Placing a useful 6 commits its other half somewhere as well. Before placing a 6–1, inspect whether the neighboring cell can accept 1. This is especially useful at the border between an exact-sum region and an equal region.
Rotate through all four directions when a promising piece seems impossible. Swapping which half points left or up can change both affected regions.
Use doubles carefully
A double such as 3–3 can fit neatly inside an equal region. Inside a different region it repeats a value and fails. Across two different regions, however, the same double may be completely valid.
Do not spend your only double simply because it fits the first empty pair. Another region may need matching values that the remaining pieces cannot supply.
Check the shape before committing
Every domino covers two cells. If your move splits the remaining board into separate empty areas, each area must contain an even number of cells.
An empty cell with only one available neighbor forces the direction of its domino. That geometric fact can be more useful than any sum clue.
A placement that leaves an isolated group of three empty cells cannot be completed, regardless of the remaining pip values. Undo it before spending time on the arithmetic.
Use contradictions as information
If a trial placement breaks a rule, remove it and remember why it failed. If there is no visible conflict but you suspect a dead end, the Hint button can check whether a complete solution still exists.
The helper offers a possible continuation. Some boards have several valid solutions, so a suggested move is not always the only correct move.
Build a repeatable practice loop
Try a few small exact-sum boards, then focus on equals and different conditions. When those feel comfortable, move to larger mixed boards. Review a revealed solution by checking how each domino connects the regions it touches.
Use the timer as personal feedback rather than a deadline. Your active time pauses in hidden tabs, and there is no penalty for taking a careful look.