Here a quick experience sharing about freehand.
I am actually a big fan of freehand on miniatures! It adds so much character and story to a miniature that it is a technique to have, train and develop with the overall miniature painting skills!
I have poor drawing skills but I try to copy some designs I like and found fitting the mini design well. Being said that while increasing my miniature painting skills I realized that I have also increased my drawing skills, especially the coloring part of it obviously.
Now drawing with a pencil and adding color on a piece of paper is quite easy (if you know how to draw) but drawing on a 3D surface with a brush and acrylic paint is another story.
Here my experience sharing about acrylic freehand:
Technically wise, the most difficult aspect of acrylic freehand is to control the paint, too much water it flows everywhere without covering anything, not enough water it dries and sticks to your brush without getting on the model. So you need to find a good balance, my first reaction was using a drying retardant, which works really well, but then I have tried Liquitex Flow improver, which is technically a drying retardant. With this medium your paint is getting really fluid. You need to add a really small amount of it in a non diluted paint. It will keep it flowing and under control without drying fast. You can even use small bristle paint brush. So then you can remove this potential frustration out and continue your freehand experiment...
I always paint my freehand after the area is finished with all the highlights and all the shadows applied. I paint on top of it then I apply the own highlight and shadow of the freehand with its own color scheme.
Then you will think that you need to be good with your drawing skills, in a way, yes it will help you, but if you are not that good, as me, no need to worry... My freehand, for the moment, are mostly based on some designs or pictures found on internet, like a drop of blood, just google "drop of blood" then you can select what you are looking for, and so on... You can use soem illustration that inspire you and modify them. You can ask a friend which has better drawing skills than you to draw you something. Then it is just a question of reproduction...
Reproducing a drawing on a model can be difficult, what you need is patience. I always start with key points, for exemple a face, I will use the eyes and other key points which will give me the overall scale. So I place 5 or 6 key points then I draw the overall structure with the main lines of the drawing. Then step by step completing the drawing, adding it shadows and highlights and even mid tones as you would do on the model itself.
You can also train on a piece of white paper, which I strongly advise at the beginning and before the first freehand experiment.
If you have any mistakes, no panic it can be covered up with the colors of the model under layers (try to keep your color scheme in a notebook so you know which colors you used).
Freehand is always a really tiring task for me... I am so concentrated that I often sweat during the process. You need to be 100% focus on the job... Normally miniature painting is really relaxing hobby for me, but freehand is not, but it is so rewarding that it really gives value to the efforts!
Here some miniatures where I have used freehand:
And many more to come!