Fingers start position
The most important thing is to place your forefingers in the middle of horizontal row. For QWERTY keyboards that would be keys "F" and "J". The rest of fingers (except for the thumb) should be placed along the "home" row. During touch typing you reach other keys starting from the base position. You do not need necessarily come back to the base position, after a key has been pressed, although, your forefingers should keep their base position whenever possible.
It is also important that your arms and hands are arranged in a natural way. Do not try to stretch them for typing! You should not lay your hands in a "rabbit fashion". If you feel somehow uncomfortable during typing, change your position so that you do not feel any inconvenience.
The correct finger-key mapping resembles a "broken cascade". It is "broken" because numeric keys (the first upper row on the keyboard) are an exception. If you have a typical keyboard e.g. QWERTY, you can try that it is further for your forefinger to reach from key "J" to key "6", than from key "F" to key "6". The "finger-key" mappings should look, therefore, like this:
Most of resources in the Internet show that fingers belong to a given key in the form of "full cascade". This approach does not work as it should in practice, although, it looks very nice in theory - in the picture:
Although the correct approach is not obvious and seems less popular, we want to teach you it. It is just more comfortable for your hands.