Usage of s-mines requires a lot of expirience. You have to know where people are used to move and where people are not used to move. On many maps there are certain paths and bottlenecks where both friendly and enemy units tend to come along. But knowing those spots isn't all. You also need to have a feeling which faction attends these spots next. And that has to do with flag cappings. People behave like lemmings. Once they've capped a flag, they move along. And even more troops will spawn on the new position. And most likely they move to the nearest enemy flag next. Accordingly you should mine the path between these two close flags while or shortly after the enemy has capped that flag. Which, of course, requires you to be at that right spot. But most important is to make sure that no friendly units are using that path for a counterattack. Then you would blow up your own men. Accordingly, being lonely is somewhat essential sometimes to make proper use of s-mines. Once there are a lot of blue arrows around you, you should not make use of them.
Another approach is to place S-mines at spots, that are only being attended when a flag is being capped. Most flags offer a few covered positions for attackers, positions where people gather or hide when capping, while the defending team does not go there. These can be spots not even far from the usual transit routes. The question is here: where would you hide if you'd be trying to cap a flag all alone? Then you know where to drop them.
Edit:// I sometimes drop mines out of the windows above the doors. If I want to leave, I take a window on ground level. Of course you should be aware that no friendly units come along.
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