1. Step: Add the ones.

$3+9=123+9=\backslash textcolor\{orange\}\{12\}$

This is more than 9. So you need to "drag the 10" one position to the left in order to "memorize" it.

2. Step: Add the tens.

Don't forget to add the "remembered" tens.

$9+1+1=119+1+1=\backslash textcolor\{orange\}\{11\}$

This is more than 9. So drag the 10 one position forward to the 100s and thus "memorize" it.

3. Step: Add the hundreds.

Don't forget to add the "remembered" hundredths.

$9+1=109+1=\backslash textcolor\{orange\}\{10\}$

Drag the 10 one step forward to the 1000s

The final result is $10121012$.