For me, freehanding text is one of the harder things to do. Getting text size, spacing, and alignment is always a paint. For my latest model, a Chaos Land Raider, I created a simple template/guide in PowerPoint, that I then used to help guide my painting of letters. Here's how you do it:
Step 1.
Measure the space where you want the lettering. I was putting this on a Land Raider side door, which was about .75 inches
Step 2.
Open PowerPoint, create a text box, and write out the text you want. Play around with fonts to get the style you want. Once you're done, right click on the text box and select 'Format Text Box'
Step 3.
First, go to the tab called 'Text Box' and set the margins. You'll have to play around with these settings a bit. I went with a 0 margin, you might want to go .01". This will determine how much room around your lettering you have to play with.
Step 4.
Next, go to the tab called 'Size' and set the width to your measurements. If your text wraps inside the box, you'll have to play around with font sizes so that everything fits on one line. Once you're done, clone your text box and stack them like I have in the first picture.
Step 5.
Print your sheet, cut out one of the text boxes (either the upper or lower box), then cut around both of them. Use some low tack tape to attach this to the area you want to paint your lettering, and as you can see, you'll have a window for painting and the lettering above or below to help guide your spacing and size. You can cut out the upper or lower text box, depending on the spacing on your model.
The really nice thing about this is that if you want to paint lettering on other vehicles, your template will help ensure everything is the same size.