When playing against an Aggro deck in Magic: the Gathering, it is important to have a solid sideboard plan to counter their fast and aggressive strategies. Here are some general tips for side boarding against an Aggro deck:

Add-in cards that can remove or slow down their early threats: Cards like Path of Peril that can wipe the board or Cut Down and Disfigure that can take out early creatures can be very effective.

Add those cards that can gain life or protect your life total: Cards like Renewed Faith, Lone Missionary, and Soul Warden, and spells or multi-purpose counterspells like Absorb can help to mitigate the damage that Aggro decks can deal.

Add cards that can disrupt their game plan: Cards like Blind Obedience and Sund Droplet or Flashfreeze can be effective at slowing down their game plan and disrupting their combos. Color-specific counterspells are usually cheaper to cast and are a solid way to stop a specific spell or creature from resolving.

Remove slow and expensive cards: Cards that have a high mana cost or take too long to set up should be removed from the deck in favor of faster and more efficient cards. This is usually done during sideboard games where the high-mana cards are replaced with low-cost spot removals and counterspells.

Balance your curve: Make sure that your deck has a good balance of early, mid, and late-game threats to match the tempo of the Aggro deck. The best example of this is curving spot removal starting from the 1st turn, Cut Down, 2nd turn Abrade, and third turn another spot removal or open mana for a counterspell.

Overall, the best sideboard plan against an Aggro deck will depend on the specific deck you are playing and the Aggro deck you are facing. However, by following these general tips, you can create a sideboard plan that can help you to counter their fast and aggressive strategies and improve your chances of success in the match.