[1] והמשנה ברורה כותב להלן שיש לגרד עוד קצת ביד ימין במקום ההפרדה "לרווחא דמילתא". (וכן הדין בנגיעות שהופרדו על ידי אשה)

[2] אבל לא אם גמר בדעתו שלא להמשיך.

The method of writing and the rules for lishmah             recorded class

When the Torah commands us to “write”, it teaches us that the “writing” needs to be done a specific way to be considered as Halachically “written.” It is unlike ordinary writing that when you simply write the letter, it is considered Halachically “written”. In reality, the “writing” is actually defined according to certain Halachic definitions of "Derech Ktiva".    
For example: letters are created by adding ink, not taking away ink, and by the Kulmus actually moving and making contact as opposed to dripping the ink or stamping.  
Similarly, if a righty writes with his left hand, a lefty with his right hand or one who writes with his mouth or the like, the writing is considered invalid. If one is ambidextrous, he should write with his right hand, but if he writes with his left hand it would be kosher. If one writes with one hand and does everything else with his other hand, it is a dispute whether or not it is valid. The custom is to accept this writing anyway.

Corrections also need to be made with the appropriate hand. Negiot (touching letters) are kosher B’dieved if separated with the wrong hand. 

One should not write if he is unable to read Hebrew and if he wrote, then there are those who say it would be kosher bideved if written without mistakes.

One should l’chatchila say each word just before writing that word. If the word is pronounced differently than the spelling, it would be articulated according to the way it is written.

A sefer Torah needs to be copied directly from another written text, while Tefillin and mezuzot can be written by heart if the sofer knows it well.

Any of the mitzvot that are required to be “written”, needs to be done with the intention of that mitzvah (L’shem…). The kavana needs to be verbalized just prior to writing. The sofer says “I am writing this sefer L’shem Kedushat Sefer Torah” (and similarly for Tefillin and Mezuzot).
If this intention was not verbalized but only thought, there are those who hold that the writing in invalid even bidieved. This verbalization is effective for that mitzvah for which he is writing, (for example, if he is writing Tefillin, the L’shem would not be effective for mezuzah). If he took a break but did intended to continue later, the original L’shem would still be effective bidieved. L’chatchila, he should say a new L’shem. All this applies if it is the same sofer, but if a different sofer takes over, then a new L’shem is needed.

If a drop of ink fell on the klaf and dried, it is not permitted to use that ink to write a letter. The letter would not be considered “written” and certainly not considered to be lishmah. If the ink is still wet, it can be drawn out lishmah to write a letter.

It would not help to write over a letter that had dried if it was not originally written lishmah.