When building encounters, start by choosing the type and number of monsters that make sense for the situation. Then use the following guidelines to compare the challenge rating of the monsters, the level of the characters, and the ratio of monsters to characters. If the quantity of monsters or their challenge rating is beyond the indicated guidelines, the encounter might be deadly. Be especially careful with potentially deadly encounters when the characters are 1st level.
Finally, tune encounters by adjusting the number of monsters, increasing or decreasing hit points, or making named or unique monsters more powerful.
Hit Points: Standard monster hit points are an average of the monster's HD range. You can increase or decrease hit points within that range to model particularly weak or particularly strong monsters. To make a fight easier, you can also treat monsters as "instant minions," ignoring their usual hit points and letting a single attack kill them. Having a few enemies die quickly can turn the tide in favor of the characters and keep a battle from feeling stale.
Named Monsters: Adding an extra attack or maximizing damage can make a named monster or unique foe more challenging. Named monsters can also be given legendary actions or the Legendary Resistance feature to make the fight more interesting.