To use the tileset: Set your map to 50px grid. Pick two textures, one for walls and one for floors. Selecting the background layer, block in the background in the wall texture. Then block in the rooms and corridors with the floor texture, making sure to stick to the grid.
Drag a tile to the background, set it to snap to grid, position it, then copy it to all similar positions. Continue with the other tiles. I usually start with the inner corners, then the walls and finally the outer corners.
Finally set your draw colour to the wall texture, set the thickness to 10, and the fill colour to empty, and draw round the walls. This will create an "in-fill" of 5 pixels that will sit underneath the bevelled edge of the tiles. Job done!