It's pretty easy actually. First, it's best to aline them from top to bottom. I'll give 2 examples. 1.2 * 2. Multiply like you usually would: 2*2 is 4 and 1*2 is 2. So now we have 24. Now we count the number of spaces from the right of the numbers to the decimal point. So there's one space from 2 to the decimal point. So now, we put the decimal in the answer after one space before the 4 giving you 2.4
Now I'll give you another doing the same. 1.2 * 2.4. Multiply like you would: 288. Now we do the same thing, count from the last number on both numbers to the decimal point: from 2 to the point and from 4 to the point. There are 2 spaces in total meaning we add the decimal 2 spaces: 2.88
I hope you understand how I explained it. It's a little hard explaining using a keyboard but I hope you were able to understand it :)