The only safe way is to manually designate one tile at a time (but, as indicated in the OP, the better way is to plan ahead so you don't have to do it like that).
If you have a single miner you can use designation priorities to usually* work correctly, but with more than one the work orders are issued in priority order, but that has very little relation to the order in which the miners arrive at the designated tiles, so you mine them out in the wrong order quite often.
Also note that a single miner is often more efficient than multiple miners in this situation, because with a single miner the designated tile is removed, the dorf rushes off, you mark the next tile, and the dorf returns (assuming the rushing off was to idle activities rather than another task. With multiple miners the next designated tile tends to get claimed by a miner far away while the first miner is still in the "rush away and don't take any new jobs" mode, which leads to long waits for miners to walk to the work scene.
*There are weird bugs when a single miner can select and mine out the lower priority tile beside themselves rather than the higher priority one they're standing on (and they should have moved away from before removing the tile). This can lead to them either plummeting down as soon as the supporting tile has been removed, or just after that as they dig away the tile they're standing on after having ensured they can no longer move to the safety of the adjacent tile.