pennon, flag, to fasten
旘 centers on a pennon or flag, and by extension the act of fastening or attaching such a banner. The meanings are closely related: a pennon is a type of flag, and to fasten is what you do with it.
旘 combines 方 (direction, side) with 戠, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with flags and pennons.
The left side 方 suggests a direction or side, and the right side 戠 looks like a complex banner with a pole and decorations. Picture a pennon fastened to a pole, fluttering in the wind.
For シ, imagine a ship (シップ) flying a pennon as it sails: ship -> シ, and the flag is fastened to the mast.