1. Using the Char Sheet, look up the rating in the skill about to be checked.
   In our example, the skill would be Athletics. Let's assume our player has a rating of 3. So the fourth column applies here.
    
2. Take a look at the "Probabilities of Success Table".
   The last two columns show different spans of percentages. 16% to 20% means the numbers in the fourth line (the second line of category "unlikely") are relevant for this Skill Check.
    
3. Find the intersection cell.
   The intersection point of column 4 and line 4 shows a "4". This means our player would need to roll a 4 or lower (1, 2, 3 or 4) on their d20 (20-sided die) to break open the door successfully.

Information on the various skills is to be found here.

Is "a 20% chance" not always "1 to 4 on a d20" (which would mathematically equal a chance of 20%)?
The "Probabilities of Success Table" already combines the objective chance of achieving the task with a wastelander's special talents.
If a fellow character is pretty strong and has a rating of "9" in Athletics, they would have to roll "6 or lower", according to the table.

So, if I have 0 SkP (Skill Points), I cannot break open the door?
If you have not activated (meaning you invested at least 1 point in) Athletics, then no. At least not with Athletics.

What else can I do then?
Maybe another skill is more appropriate? With a successful check in Gathering Supplies you might find a suitable tool nearby.
A successful skill check in Science or Handcraft might reveal the weakest point of the door.
Or a magic spell temporarily weakens the structure of the lock or wooden planks for a short amount of time.

What do the numbers with an asterisk (*) in column 1 mean?
As soon as an event is more than "unlikely" to happen (equalling a chance of 21% or more), there is the possibility of beginner's luck.
Had the GameMaster described another, weaker structured or more rotten door, the chances to break it down could have been numbered 62%.
In this case, even a character with 0 SkP in Athletics could try. And a "1 or 2 on d20" would be sufficient to break it down.

Optional rule: What is an "blind roll" (or "open roll")?
A GameMaster might ask players for an "blind roll" to identify HOW successful a check has been. Let's create another, a bit more sophisticated example for that.
A bunch of wastelander are out in the woods, searching for food. Each player can choose which skill they would like to use. Food or Gathering Supplies are the more obvious choices. But they could also count on Intuition or Awareness, just to meet their personal strengths.
The GameMaster now asks them for an open roll. Player A rolls "19" on Gathering supplies (with only 2 SkP). A quick consultation of the table reveals to the GameMaster that this attempt only succeeded if it were "almost safe" to find food.
However, Player B is more fortunate and rolls "3" on Food (4 SkP), meaning a success even under "very unlikely" conditions.
Player 3 rolls a "12" and has 3 SkP in Awareness. This equals a "proper chance".
Of course, it will depend on the forest (and therefore on the GameMaster) if there is any food to be found at all in a special area. So the players can never insist on a certain outcome. Actual results of Skill Checks are always up to the GameMaster.
We recommend the rule of "blind rolls" especially for checks in Magic Skills since their success might show up eventually and not immediately.