Amy at Cloth Habit wrote an awesome post about pattern drafting for the hobbyist with Adobe Illustrator. If Illustrator is out of your budget, you can try pattern drafting using Inkscape, which is free vector graphics software that can be used as a substitute for Illustrator.
Most of this post will not make sense to you until you play around with Inkscape for a while. To get started in Inkscape I suggest you first go through some of the tutorials available in the Help menu. There are lots of tutorials and help available online too, including a very complete Guide to Inkscape.
When you get started, first make sure you get familiar with
The pen tool
Edit paths by nodes, (especially join selected nodes and break path at selected nodes)
Combine and break apart paths
Object selection methods
The Transform commands
Rotating objects and moving object rotation centers
Since Inkscape is a free program, of course it does have some bugs and quirks you need to watch out for. For free software, however, it really is pretty powerful and capable. But save often!
Things to watch out for:
Don’t print directly from Inkscape; the scale will not come out right (as of version 0.48, anyway). Instead, save a copy your of your document as a PDF (from the menu, File > Save a copy, select PDF as the file type). This will save whatever is within your page layout area to a PDF. Now you can print the PDF from Adobe Acrobat Reader and it will come out fine. Another printing issue you might need to watch out for, which is not related to Inkscape, is the print quality setting on your printer. My inkjet printer will not print to an accurate scale unless I set it to the highest quality print settings, so any time I print PDF sewing patterns, whether I’ve made them or not, I have to use the “presentation” print settings.
When you use the Scale command (from the menu, Object > Transform, then click the Scale tab), Inkscape scales the object based on what you see on the screen, so the width of the stroke will be included in the calculation when you scale an object to a specific size. For example, if you have a circle with a 1 mm thick stroke and scale it to 2 cm, then draw a line that is the length of the diameter of the circle by snapping from quadrant to quadrant, the line will be 1.9 cm long, not 2 cm. So what I do when I’m scaling an object is this: select the object, remove the stroke (if you don’t have any fill on your object it will temporarily disappear, but don’t worry, just make sure you don’t de-select your object), scale the object, then add the stroke back.
To measure the length of a line (path) you have to use the Measure Path extension. From the menu, Extensions > Visualize Path > Measure Path. This will actually print the length of your path as text in your drawing. It’s a little clunky, but it works.
Understand what is considered a single path. You can have a single path with multiple segments that don’t even touch each other (to make multiple paths into one, Path > Combine). If you select a node on a path and click Break path at selected nodes it is still a single path. In order to break it into two paths, you need to also break it apart (Path > Break Apart). However, if you have two paths and you use Join selected nodes to join the endpoints together you will now have one path.
To add seam allowances you will need to use the Outset command. Linked Offset and Dynamic Offset will not do what you want. First make sure that you have joined all the nodes together in your path – Outset may not do what you were expecting if you’ve only combined multiple segments. Here’s the weird part: you have to specify the inset and outset in pixels (abbreviated px), not inches or centimeters. There are 90 pixels per inch, or 35.433 pixels per centimeter, so you’ll have to do some calculations. Let’s say you want to add a 5/8” seam allowance. From the menu, select File > Inkscape Preferences. Scroll down and select Steps. Change Inset/Outset by to 56.25 px (.625 inches * 90 px per inch = 56.25 px). Duplicate your path that is the stitching line for your pattern. The copy will now be selected. From the menu, select Path > Outset. The Inset and Outset commands treat every path as if it was a closed shape, so you may need to add segments to the ends of the copy of your original path (or close it) in order to get the outset command to do what you want. The resulting offset path will always be closed, so you may need to trim it.
There is no true trim command like you would find in a drafting program. You have to get creative in order to accurately trim paths, and often it takes a few extra steps. Sometimes I will draw a temporary line, snapping one end to the intersection of where I want to trim my lines, then I add a node to the path I want to trim, snap the new node to the end of my temporary line, delete any extra nodes on the line I’m trimming, then delete my temporary line. Whew.
1-2. Draw a temporary path, snapping one endpoint to the point you want to trim to. 3. Add another node on one path near the point you are trimming to. Select the node, and snap it to the endpoint of the temporary path. 4. Select the extra nodes on the path to be trimmed and delete them. 5. Repeat steps 3-4 for any additional paths you are trimming. 6. Delete the temporary path.
Sometimes the Difference command works for trimming, but it’s buggy, so save first. Draw a closed shape around the portion of the path you want to trim. If you want to trim multiple paths, make duplicate copies of your closed shape before you use the Difference command, because you have to trim the paths one at a time, and the closed shape is deleted in the process. If the closed shape is not the last thing you drew, select it and press the Page Up key until it is above the line you want to trim (whichever object is on top is the one that will do the cutting). Select both the new closed path and the path you want to trim, then from the menu, Path > Difference. The Cut Path command will often work if Difference isn’t working. Then you can just delete the extra path created when the original path was broken into two. Sometimes Cut Path will (randomly and unexpectedly) do what the Difference command should have. If the wrong half of your path is being cut off, try reversing your path before you cut it (Path > Reverse).
If you scan a paper pattern at 90 DPI and import it into Inkscape, it will be the right size. If you scan with a different resolution, you will need to scale the image after you import it. For example, a 300 DPI scan will need to be scaled 90/300*100% = 30%.
I find the easiest way to trace a scanned drawing is to simply use the pen tool to draw a path with straight line segments, placing a node in the center of each curve. Then go back and delete the extra nodes, which changes those path segments into curves. After that, select and move the handles on the nodes to change the shape of the curves. Add more nodes if you need to. It takes surprisingly few nodes to match a shape
I’m willing to answer questions about Inkscape. Please be as clear as possible with any questions you have and include links to screen shots if possible.