Now multiply the first number with the $1^{st}$ digit in $2^{nd}$ number to get intermediate results. That is Multiply 345 with 3. Write the result 1035 at the end leaving 0 spaces to the right like this.
Now multiply the first number with the $2^{nd}$ digit in $2^{nd}$ number to get intermediate results. That is Multiply 345 with 4. Write the result 1380 at the end leaving 1 spaces to the right like this.
Now multiply the first number with the $3^{rd}$ digit in $2^{nd}$ number to get intermediate results. That is Multiply 345 with 1. Write the result 345 at the end leaving 2 spaces to the right like this.