A horizontal line in the score indicates that the note requires the use of a legato line (also called a portamento line or portamento).

When using legato lines, you need to smoothly transition from one note to another to create a smooth musical line. Specifically, you can use fingering techniques to simulate this transition effect when playing the flute. Here are some possible methods:

1. Use the portamento technique: As you play one note, quickly adjust the shape of your lips and mouth to the shape of the next note, creating a smooth transition. For example, if you want a smooth transition to a D note after playing a C note, you can slightly adjust the shape of your lips and mouth to the shape of the D note to create a smooth transition.

2. Use the vibrato technique: when playing a note, quickly vibrate the shape of the lips and mouth to produce a brisk, rhythmic transition effect. For example, if you want a smooth transition to a D note after playing a C note, you can quickly jitter the shape of your lips and mouth from the shape of the C note to the shape of the D note, creating a smooth transition effect.

It is important to note that the length and shape of the legato lines may vary depending on the style of the piece and the style of the performer. Therefore, when playing a piece, it is best to follow the specific instructions in the score and choose appropriate techniques and expressions based on the style of the piece and the requirements of the performer.