It's mainly used for detailed work like touching up and working in small areas. Follow these steps to use a margin trowel correctly.
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Mix mortar using the margin trowel vigorously for 2 to 3 minutes. It should be free of lumps with a smooth consistency. Wash the margin trowel immediately after use to prevent the mortar from drying on it.
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Scoop out mortar with the margin trowel. Spread it in narrow places where a regular trowel will not fit. This is especially convenient for areas along the bath tub and around fixtures such as plumbing.
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Spread mortar on the back of small pieces of tile. This needs to be done if the back of the tile is notched in order to get the mortar in the small crevices. This is referred to as "back buttering."
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Touch up hardened grout lines with a margin trowel. Gently scrape away the grout in a smooth motion. Don't dig because you can damage the tile.
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Use a margin trowel to separate wet concrete from a forming board when pouring concrete steps. Once the concrete is partially dry run the margin trowel along the edge to separate it.