Note: I'm using the Gimp on Ubuntu, I imagine Photoshop or most image editing programs will be similar.
Step 1. Find a suitable picture. This is where the art is, you need something of a side view that has limbs separated and will give a pretty clear idea of what the figure is and what it's doing in silhouette. See this post for a possible place to look. I'll use this as an example:
It's not a particularly good pose, but the spear will be clear.
Step 2. Trim the image. Use the rectangle select tool to select enough space around the figure to give you some buffer and then go to the Image menu and select Crop to Selection.
You should end up with something like this:
Step 3: Homogenize the background. Use the color picker tool (you can see me hovering over it in the picture above) to pick one of the background colors. Use the paint bucket tool to fill in the background with that color. Now use the paintbrush tool to clean up any left over background details.
Step 4: Select that homogenized background. Use the select by color tool to pick the background color.
Then go to the Select menu and pick Invert. Once you've done that you'll see something like this:
What that does is select anything that's not the color of the background. But, since we've homogenized the background, it selects our figure for us.
Step 5: Paint the figure black. Without deselecting the figure, use the paintbrush tool to paint the figure black, giving you something like this:
Notice our figure isn't solid black, we need to take care of that. You could just carefully and laboriously paint it all with the paintbrush, but this is supposed to be quick.
Step 6: Homogenize the background again. This time use the paint bucket tool to turn the background white.
If there are any breaks in the line of the figure the white might invade it. If that happens, like with the arm here, just undo the bucket fill, use the paintbrush tool to "seal" the holes with black and then bucket fill the background white again.
Step 7: Repeat step 4 and 5. So, select the white background with the select by color tool, invert the selection and paint it black.
Since we turned the background white, the beige in our figure is not selected and we can finally have or silhouette.
Step 8: Resize your image. I'm sure there's a smart way to do this, but I just reopen a figure I use as a benchmark and scale this new one to that.
Go to the the Image menu. Select Scale Image and I always use the drop down menu to select scaling by percentage. Resize it by trial and error. Save it. I always save as png, although it results in much bigger files than jpg, it's a lossless format, so you don't lose any info in case you want to touch it up later.
And that should give you a silhouette like this: