As a general rule, a polyfill insert shouldn’t be more than about one inch (~2,5cm) larger than the pillow cover. For smaller covers (14”x14”(~36x36cm) or less) the insert should ideally be no larger than the cover. If you are using feather/down inserts, you may be able to use inserts that measure as much as 2 or three inches bigger than the cover depending of the ‘loft’ or volume of the insert. Source
When I sized my duvet from the cover I made it 3 cm smaller than the cover.
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For me inspiration and the finished item is the most important. Patterns, photos, tutorials, and the like has helped me understand a lot about making useful items. Patterns can be useful to me as a beginner to see and learn to analyse the hows of sewing.
WP Sewing This helped me in understanding some of the many techniques used in sewing:
Hem
Zigzag
Seam allowance
Selvage
Felled seam
among many more that I have yet to explore.
Get to know your machine. How does it sound when working properly? Try to close your eyes while sewing and memorize the sound. If something is not right you can often tell by unusual sounds from the machine. Make sure to service your machine regularly to avoid problems.
Factors that influence on you work:
lightning
state of the machine and needle
right needle to the material being sewn
organizing (a good table/floor to cut on, doing things in the right pace/order, etc.)