This question specifically asks for resources, but I believe there are no such resources (like Web sites or books) simply because the writing rules are fairly simple. My answer will focus on these rules.
For practicing, check Top Five Videos You Must Watch to Learn Thai playlist by Learn Thai with ThaiPod101.com on YouTube. Here are some direct links (some content may overlap in several videos):
So the rules are:
- Each glyph has a little circle which denotes where to start writing from. Notable exceptions are:
ก (no circle) written from the bottom-left;
า (no circle) written from the top-left;
ห (two circles), written from top-left;
ใ (two circles), written bottom-to-top, just like the
ศ (extra stroke) the extra stroke written last, center-to-upper-right;
แ (two strokes), written top-then-bottom and left-then-right, correspondingly;
- upper/lower vowels written after the consonant:
ดี = ด+◌ี;
- tone marks written last:
นั่น = น+◌ั+◌่+น;
- tone mark also written after the lower-vowel, too. See the note on Unicode below;
- left vowels written first (tone mark last):
แม่ = แ+ม+◌่;
- combined vowels written left-to-right:
เก้า = เ+ก+◌้+า;
Note: in order to Unicode fonts display the text properly, the rule "tone mark goes after the upper/lower vowel" is not to violate; some powerful text processors would correct the input mistakes, but others don't. The incorrect order leads to various problems, including the incorrect visual rendering and context search failures.
Also, in handwriting many native Thais write upper-vowels, lower-vowels, and tone marks the last, just like in European languages some write accents and diacritical marks over the vowels after writing the entire word. As a language learner, I find it not convenient because (1) by the moment I wrote the word/syllable, I tend to forget those; (2) mind the Unicode, it's too hard to follow two different rules for handwriting and for the keyboard input.