So...one of the major issues I have as an artist is that I don't plan well before I start making something.
My natural creative process is very intuitive which is both good and bad. But, it is especially bad when I want to reproduce something I have sewn before that does not have a pattern.
That is why I have started making sure to draw out a pattern before or while I am creating something. Because it is so much easier that way!
However, if you do have to re-make a plush animal and don't have a pattern there are a few things you can do to make it easier on yourself assuming that you have a picture to work with. This method will also allow you to enlarge or shrink your pattern while keeping the correct proportions. It works best with simple designs.
Step 1:
Find a picture that shows the front or back of your plush critter.
You can either print this picture or open it up in a computer program such as Paint.
Step 2:
Draw a box around your animal, matching it's height and width.
Measure the width of the box and divide it in half to draw the mid-line, and do the same with the height.
Already you can start to see how the negative spaces start forming angles with the sides of the box.
Step 3:
Draw straight lines along the large angles of the animal so that they extend past the box. A protractor can be used to actually get the degree of the angles if you want to be super accurate :)
(Here is a picture with the angles filled in, making them easier to see.)
Step 4:
This is not super necessary but I also like to draw a straight vertical line from the bottom corners of the animal up to the top of the box and from the top corners of the animal down, to show the relationship between the width of the head and bottom.
What you can do now is draw a box on a piece of freezer paper (keeping the height and width proportions the same), draw your mid-lines and use the angles you have discovered to make a much more accurate pattern for your animal.
This is actually the same basic method some art teachers use for beginning drawing, because it begins to teach how to measure angles and relationships within a composition. Hope this helps a little bit!
