Learn the גמרא דף יא. “ורמי” (second line from bottom of the Amud) until the two dots on דף יא עמוד ב
Conclusion of Topic: צורת הפתח מן הצד
We discussed three opinions in the Rishonim (רש”י, ר”ח & ריף) for what is the case of צורת הפתח מן הצד
Learn רשב”א who asks multiple questions on the explanation of the ר”ח and ריף with a possible answer for them
https://hebrewbooks.org/pdfpager.aspx?req=37275&st=&pgnum=15
According to Rashi if the top string is wrapped on the side of the pole at the top is it acceptable?
Learn ט”ז ס’ שס”ג ס”ק ד
Is a צורת הפתח made by placing the top string on in between a groove on top of the vertical post acceptable?
Learn שולחן ארוך או”ח ס’ שס”ב סע’י יא
See and משנה ברורה ס”ק סד which discusses the two halachos above and other details of צורת הפתח מן הצד
https://hebrewbooks.org/pdfpager.aspx?req=60389&st=&pgnum=54
https://hebrewbooks.org/pdfpager.aspx?req=60389&st=&pgnum=55