Have you never added 2-digit numbers?
35 +27 = (30 +20) +(5 +7) = 50 +12 = 62
35/100 +27/100 = (35 +27)/100 = 62/100
0.35 +0.27 = 0.62
The basic idea, taught in 3rd grade, is to line up the decimal points of the numbers, add enough zeros on the right to make the numbers all have the same number of digits to the right of the decimal point, then add in the usual way. The sum has the decimal point aligned with the rest of the numbers.