When you're learning to add and subtract decimals, graph paper comes in handy. Graph paper will help you keep decimals and place values in order. If you need some, you can print graph paper for free online.
The most important thing is to keep the decimal point in the right place for each number you're adding or subtracting. When the decimals are lined up properly, everything else falls into place.
If you like, can also add zeros as placeholders (for example: 5.2 can become 5.2000).