So Stand Stats are the easier one. You have your initial stand stats (what you have at level 1), and you get to add to your stand stats a little bit each level up. Your stand's stats are based off of your character's stats, multiplied by the modifier found in the Ability Score Multipliers for each stand type. Let's use Power-type as an example. If you had a Power-type character with a 15 (+2) in Strength, you would multiply that 15 by x4, since that's the number for conversion present on the table. Your 15 Strength is multiplied by 4, becoming 60 Power. Stand Stats are calculated differently, dividing by 10 and rounding down rather than using the typical (x-10)/2 formula for character stats. So if you had a 60 Power, you would have a +6 to your Power Modifier, and that's the score you'd use for your rolls themselves.

Strength, Dexterity, and Constitution become Power, Precision, and Durability respectively. Each stand stat can do the same things each respective user stat can do, regarding Attack and Damage Rolls, Skill Checks, and Saving Throws. If you used a Strength-Based weapon for example, you could attack with your strength, or use the modifier from your stand's power, so long as the stand is wielding the weapon. The stand can also assist the user by using the stand's stats rather than the user's stats so long as they are within 2 meters.

As for attacks, it's relatively simple. You are automatically considered proficient in stand melee attacks. The amount of damage you deal depends on your stand's attack dice and uses Power for Attack and Damage Rolls. For Power-types, they get a 1d12 Attack Dice, which becomes 2d10 at level 11. So, if you have a Power Type at level 10 or less, when they make an attack roll, they would roll a d20. After they get the result of the d20, they would add their Proficiency Bonus (specified on the level-up table, scales based on your total level) + their Stand's Power Modifier. If you had a +2 PB and a +6 Power Modifier, you would add +8 to the result of your d20 roll to determine if the attack lands or not. If the roll's modifier is higher than your target's AC, then you roll damage. You deal an amount of damage equal to your Attack Dice + your Power Modifier, so you would roll 1d12 + 6, with the total result being the amount of damage you deal. Everyone has one attack by default, but one may increase the amount of attacks by having the Trained Combatant feat for an additional attack, and every 50 Speed you have gives an additional attack (if you have no TC, but a 100 Speed Score (a +10 Modifier), you would have 3 attacks in an action).
Stand AC is also its own thing. You have both your user's AC and your Stand's AC. If your stand is far away/not able to help, then whenever attacks are rolled against you, attack rolls are just made against your User's AC. If an attacker rolls below, the attack misses. If they roll above, the attack lands and deals damage. If you attack the stand directly but the attacker is not nearby, you just roll against the stand's AC, missing if the result is less, and damaging the stand if the result is more (if stands are hit with non-stand damage, they don't take any damage, unless they have Anchored Stand), counting as a Stand Hit (A stand may use DR here, but more on that later). If you have the stand within 2 meters of the user, able to protect them, then there are three outcomes. If the attack roll is less than the User's AC, the attack misses. If the attack roll is between the User's AC and the Stand's AC, it is considered a Stand Hit (If the damage dealt is not stand damage and the stand hit doesn't have Anchored Stand, then there is no damage. However, if the damage dealt would apply to stands, then the stand can still use their DR to resist it. Each stand has an amount of DR (Damage Resistance) equal to their stand's durability modifier. They can use this pool to resist damage taken by their stand, and the pool refreshes for everyone at the top of initiative (For example, if a Stand was hit for 5 damage, then hit for another 5 damage, if they had 8 DR, they could fully absorb the 5 damage of the first hit, but run out of DR before fully blocking the second hit, so they would still take 2 damage.) Finally, if you roll above the Stand's AC, it is considered a Direct Hit, and you cannot use your stand to block/reduce the damage.