Transferring a design onto your fabric can be tricky, but having the right marking tool for the job makes a huge difference! You can use a light box (or window) to transfer any line drawing on to fabric, but which is the best pen to use?
Leaving a Permanent Mark...
If you want a very clear, precise line that's going to last the length of your project, you can't beat using pens that leave a permanent mark. Of course, you have to make sure that you will stitch over the design, completely covering the lines, otherwise they will show through in your finished work.
You can use gel pens (including white) or pigment pens to trace over your design leaving a clear precise line to stitch over. I use Gelly Roll pens (White) and Staedtler Pigment Liners (Black) 0.2 & 0.7 mm
Iron it off...
If you want to use an eraseable pen the best ones are Frixion pens either roller ball (thin) or colouring pens (thick), they are both available in a range of colours suitable for marking most colours of fabric. If you want to make an eraseable mark on black, or very dark fabrics then you'll need to use a Clover Sashiko Marker Pen (White). These are designed for marking Japanese Sashiko Quilting patterns, and they leave a clear, precise line. The only drawback is the line doesn't show up until the ink is dry, so you have to be patient. The line appears after a couple of minutes, and gets more intense over about an hour or two.
These are both heat/steam erasable, so you just need to hover an iron over the fabric and the line disappears, so MAKE SURE YOU IRON YOUR FABRIC BEFORE MARKING IT! They also leave a mark (like a shadow) on silk and viscose, so test them on a scrap piece of fabric first.
Oh, my it's Humid...
Water (Turquoise) and Air (Purple) erasable pens are best saved for dressmaking, where the whole garment is designed to be washed, and the marks aren’t required for long periods of time. Turquoise marks can be hard to remove without washing. Both types of pen will fade due to the humidity in the air over hours/days (Air erasable) or months (Water erasable). They also make a thick mark which is not accurate enough for small embroidered motifs.
Back to the Good Old Days...
Sometimes simple, old fashioned techniques work the best - yes, you can just use a pencil to mark your design! A good old HB! Bear in mind though, that although it will rub off paper, it won't rub off fabric, so you'll need to stitch over your design completely.. And for dark fabrics? A sliver of soap leaves a clear and instant mark. Soap will wash out, but it also disappears over a few months due to the humidity in the air, and can also rub off if you handle the work too much. Tailors chalk is handy for temporary marks on silk and viscose.
A fine gel pen if I'm sure it will be covered by stitching, otherwise I'll go for a heat eraseable Frixion pen or Sashiko marker.