erasing pieces looks the easiest. Draw the signature, then add keyframe, delete a piece, add frame, delete a piece... and so on then when there is no pieces to draw select all keyframes and reverse them (right click menu on windows)

erasing pieces looks the easiest. Draw the signature, then add keyframe, delete a piece, add frame, delete a piece... and so on then when there is no pieces to draw select all keyframes and reverse them (Right-click > Reverse Frames)