Align the numbers by their decimal points.

Adding the thousandths column ($0+0+9+9+9$):
$0+0+9+9+9 = 27$. Write down 7 and carry over 2.

$$\begin{align*} &\quad 30.90\overset{2}{0} \\ &\quad \phantom{0}3.090 \\ &\quad \phantom{0}0.309 \\ &\quad \phantom{0}0.039 \\ &+ \quad 0.009 \\ \hline &\quad \phantom{00.00}7 \end{align*}$$

Adding the hundredths column ($0+9+0+3+0+2$ (carry-over)):
$0+9+0+3+0+2 = 14$. Write down 4 and carry over 1.

$$\begin{align*} &\quad 30.9\overset{1}{0}\overset{2}{0} \\ &\quad \phantom{0}3.090 \\ &\quad \phantom{0}0.309 \\ &\quad \phantom{0}0.039 \\ &+ \quad 0.009 \\ \hline &\quad \phantom{00.0}47 \end{align*}$$

Adding the tenths column ($9+0+3+0+0+1$ (carry-over)):
$9+0+3+0+0+1 = 13$. Write down 3 and carry over 1.

$$\begin{align*} &\quad 30.\overset{1}{9}\overset{1}{0}\overset{2}{0} \\ &\quad \phantom{0}3.090 \\ &\quad \phantom{0}0.309 \\ &\quad \phantom{0}0.039 \\ &+ \quad 0.009 \\ \hline &\quad \phantom{00.}347 \end{align*}$$

Adding the ones column ($0+3+0+0+0+1$ (carry-over)):
$0+3+0+0+0+1 = 4$. Write down 4.

$$\begin{align*} &\quad 3\overset{1}{0}.\overset{1}{9}\overset{1}{0}\overset{2}{0} \\ &\quad \phantom{0}3.090 \\ &\quad \phantom{0}0.309 \\ &\quad \phantom{0}0.039 \\ &+ \quad 0.009 \\ \hline &\quad \phantom{0}4.347 \end{align*}$$

Adding the tens column ($3+0+0+0+0$):
$3+0+0+0+0 = 3$. Write down 3.

$$\begin{align*} &\quad 3\overset{1}{0}.\overset{1}{9}\overset{1}{0}\overset{2}{0} \\ &\quad \phantom{0}3.090 \\ &\quad \phantom{0}0.309 \\ &\quad \phantom{0}0.039 \\ &+ \quad 0.009 \\ \hline &\quad 34.347 \end{align*}$$

The sum is 34.347.