My mom is an artist, and I have been lucky enough to have been exposed to all kinds of random knowledge because of it. She explained to me how to blow up a pattern using the grid system. A lot of art books use the same technique when making murals, but done on paper rather than walls, so I had known about the concept since I was a young child.
To blow up a pattern get graph paper that is the proper size for the full-sized garment. Then square by square you copy off the pattern. If you are using a computer to grade the pattern do that before you blow it up. In the beginning I was doing it all by hand. Big hint here: A very inexpensive way to blow up patterns is to use wrapping paper. A lot of it has one inch grids on the back, and it goes on sale right after the holidays.
During my Junior year in High School I had a lot of dresses that had been my grandmother’s and aunt’s from the 1940s. I made some 1940’s reproductions based on originals, which taught me how to grade a pattern that I created from the originals. Grading is what you do to a patten to resize it to fit your size, like changing something from a size 7 to a size 14. Altering is taking the pattern or garment and changing it to take into account the quirks in your body. I used paper bags and made patterns from the dress by just pinning it and tracing around each piece. Then I graded the pattern and then altered it to fit me. Detailed explanations of grading can be found online.
To alter a pattern you take body measurements and add or subtract those measurements to the edges of the pieces of the pattern. During this process you need to account for both seam allowance and undergarments while making sure that the seams still fall in the appropriate places. Here are some tips on altering. I like drawing out the graded (sized) pattern I drafted on the material I am cutting and then alter it with chalk before cutting it. Proper altering is what the garment fit well, giving a tailored look, which is very important to spot-on18th century clothing. Without proper altering clothing has a costume feel. Note: I buy chalk by the box. If you are not buying a box, I would advise getting the pack with white and blue chalk. This year I learned about using blue chalk for white fabric… wonderful stuff. No more using pencils!