You must follow the normal timing permissions and restrictions for the exiled cards. You can't play lands this way unless you have land plays available.

Even though the card is named after a specific character, controlling any commander will satisfy its condition.

Jeska's Will isn't a mana ability, even if you select the first mode (or both modes). It uses the stack and can be responded to.

Once you've announced that you're casting a spell, players can't take any actions until you've finished doing so. Notably, opponents can't try to remove your commander to change how many modes you may choose.

There's no extra bonus if you control more than one commander.

Any of the exiled cards you don't play will remain exiled.

Once you've chosen both modes for the spell, it doesn't matter whether you continue to control a commander. This is true even if you somehow no longer control a commander as you finish casting the spell.

Use the number of cards in the target opponent's hand as Jeska's Will resolves to determine how much to add.

The commander you control doesn't have to be your commander.

You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.