i cant help you on double, Sorry, i am sorta in a rush.

But here is a print int function, maybe you can get an idea from it (recursion on 8051, ack!!) :

sprintint(char* s, int i){
if(i==0) return;
sprintint(s, i/10); //send a higher digit first
i %= 10;
addintTochar(s, i); //before printing this one
}

Ideas:
Logarithm in base 10 will give you how many digits to print on the left of the decimal. :/
Be sure to cut the recursion short should you follow how the above function works
Be sure to make sure the function can print a decimal-only number, unlike an int, a double generally doesn't truncate.