Raising attributes
I put this on reddit too:
This is effective as of May 13 2024.

To increase strength and a melee weapon skill (one handed or two handed), hit a person with the weapon. Animals do not work. Dead bodies do, but despawn rather quickly. You gain 5 points of strength experience and 10 in the weapon type. I have not tested to see if blocking with a weapon counts.

To increase agility and archery, shoot a person or an animal with an arrow. Dead bodies count. Animal corpses last a long time (I have fired 120 arrows into a body and it was still there). You gain 15 agility experience and 30 archery.

To level shields skill, block attacks from people or animals. You gain 2 experience per block. Someone did a quick test and equipping a shield did not change to damage they did, so the damage boost the skill lists is confusing (is it applied even without a shield? Is it a damage reduction for the shield? Does it just not work?).

You can also use books on yourself or your villagers. The book needs to be in your inventory. The you go to your attribute screen (or your villagers attribute screen). Each attribute you have a book for (there are some restrictions) will have a plus sign next to it. Click the plus to start reading the book. For villagers, they must be awake and there might be a range factor. Each attribute also has a maximum level (your skills max at 10, villagers vary but the number is visible when looking at their attributes page).

Apprentice books can be used at any attribute level and give 500 experience when completed (you will see a buff below your attributes and a timer). Dying before completing the book removes the effect. I have not tested to see if dying negates all the book experience or only the ‘unfinished’ portion. There is a book seller in Haerndean. His stock of books changes daily. He sells books for 150 gold, and buys them for 50. Like other vendors, he only buys/ sells limited amounts of books each day.

Journeyman books require an attribute level of 3 to use. They give 1500 experience. There is a book seller in Padslow. Books cost 300 gold, and he buys for 150. Like the other seller, how stock changes daily and he buys/sells limited amounts.

Higher level books exist as quest rewards (and potentially as loot, but I personally have not seen looted books above apprentice level yet).

For both fighting and books, there are foods you can eat to increase experience gain. For books, the food must be in effect when you start reading the book.

Cloudberries: give +50% experience gain. There is a vendor on the SE corner of the lake east of Padslow that sells them. They deteriorate fast in storage (like 2 hours).

Fish stew: gives 20% experience gain and a productivity bonus. The same vendor as cloudberries sells them. You can also make this in the cauldron (requires water and garlic /and raw fish and 5 cooking I think). There is no fishing so raw fish must be purchased.

Cranberries: gives a 30% experience boost. Can be found in the swamps north of Blackridgepool (if you find the shrine, there are some just east and south of the map marker).

Apples: give 20% experience boost. You can buy apples from the merchant in Blackridgepool.

There are other foods as well (I have gotten cake as a drop from bandits a couple of times which gives +50% experience boost), but I do not know of reliable sources for them.

You can mouse over the experience bar for attributes to see your current experience points (and the amount needed to level up).

I have done the following a few times: Eaten fish stew (and bought 3-5 extra) then a cloudberry and a cranberry). The extra stew to eat before the stew buff expires to keep all 3 food buffs active (stew is 7 hours, or 5 in winter with the -30% food duration debuff). Then I set out with 120 arrows. I killed a deer or boar (wolves work too, but I did not time the body duration for them) and did NOT loot the body to keep it around. I fired all my remaining arrows into the corpse (this takes me around 5 minutes real time), then I loot and head home. 120 arrows with the 100% bonus from the 3 foods means I gain 7200 archery and 3600 agility experience doing this.

To go from level 0 to 1 = 600 experience.

To go from level 1 to 2 = 1200.

To go from level 2 to 3 = 2999.

To go from level 3 to 4 = 4800.

To go from level 4 to 5 = 7198

To go from level 5 to 6 = 9600.

To go from level 6 to 7 = 14578.

To go from level 7 to 8 = 23802.

To go from 8 to 9 = 28800.

To go from 9 to 10 = 43214.