There's no perfect tool or method to transfer a design to fabric - only a "most preferred" tool for specific situations. When I'm transferring a design onto dark fabric that has a loose enough weave for light to shine through (thus allowing me to see my design underneath the fabric), I like to use a white water-erasable fabric pen.
The pen I use for this tutorial is from a company called Kearing, but you can find similar products from Clover, Dritz, and other brands, by searching for "white water-erasable fabric pen."
In these step-by-step photos, I'm not transferring - I'm just drawing a line on the fabric to demonstrate how the pen itself works. For a demonstration of me transferring an actual design, I've included a full-length tutorial at the end of this post!
One thing to keep in mind before diving into any sort of transfer method is that all fabrics react a bit differently to both the marks themselves, as well as the removal process. Unless you're using a transfer method that comes with a kit (in which case I will have already done the testing for you), make sure you test this or any other transfer method on a scrap piece of your fabric first!
Here's what my pen looks like - one thing to keep in mind is that these pens can be fairly expensive at about $3, and they are quick to dry out! If you're not currently drawing, make sure the cap to your pen is secured tightly to help lengthen its life.
In this next photo, I've just drawn a curved line onto my fabric. You can see that the mark is barely visible -- don't worry, though! As the ink dries, the mark will become more visible. When transferring, make sure you're going slowly, keeping in mind that it will take several seconds for the lines to fully appear. This effect is even more apparent in the video included at the end of this post.
As you can see in the image below, the line became much more visible as the ink dried onto the fabric.
Once you're ready to remove your marks - either because you're finished stitching, or because you made a mistake and want to start over - remove the mark with water. In the photo below, I've submerged my fabric and hoop into a bowl of water; alternatively, you can run your fabric under a faucet.
If you do choose to soak, as I've done here, make sure not to leave your hoop in water for too long, as it can cause the hoop itself to warp and/or the hardware to rust.
In some cases, soaking or running your hoop under water isn't enough to remove these marks. In the photo below, I'm using a cotton swab to gently agitate the fabric, which allowed me to more fully remove the mark. In some cases, I'll also add just a tiny drop of detergent to the cotton swab to further aid in removing any stray marks from a hoop.
A totally clean hoop! All that's left to do now is wait for my fabric to dry.
Here's a short video where I demonstrate transferring a design using a water-erasable transfer pen; I hope this has been helpful to you!