For  a CR, do you take the level of one character and match the monsters to that level of CR, or do you add up all the PC levels and then match the monsters?

for context, im running a campaign for two lvl5 characters, do i match the monsters to a lvl 10 threat, with a lvl ten monster, or match the CR to lvl ten but using lvl five and below monsters?

Pls give some advice here. I don’t want to kill the characters, but i also don’t want it to be to easy.

Another thing-somebody lost interest so there will only be me and one PC

I believe a monster with a CR of 10 is meant to go up against a party of 10th level players, so CR is meant to match the overall party level. 

Per the ‘24 rules it’s not about CR. You have an xp budget per how hard you want the encounter to be. (Which is kind of CR as xp per creature is based on CR.)
Theres a chart.

https://www.dndbeyond.com/sources/dnd/dmg-2024/creating-adventures#CombatEncounters

That said, the rules for combat encounters manage to be both complicated and not very accurate. "Total CR = 1/2 of total party levels" is a decent rule of thumb for challenging but not impossible encounters in tier 2.

Quote from SydneyHofferson00762 >>

For  a CR, do you take the level of one character and match the monsters to that level of CR, or do you add up all the PC levels and then match the monsters?

for context, im running a campaign for two lvl5 characters, do i match the monsters to a lvl 10 threat, with a lvl ten monster, or match the CR to lvl ten but using lvl five and below monsters?

Pls give some advice here. I don’t want to kill the characters, but i also don’t want it to be to easy.

The answer is neither of those things, exactly. The definition of Challenge Rating in the Player's Handbook says this:

Challenge Rating (CR) summarizes the threat a monster poses to a group of four player characters. Compare a monster’s CR to the characters’ level. If the CR is higher, the monster is likely a danger. If the CR is lower, the monster likely poses little threat. But circumstances and the number of player characters can significantly alter how threatening a monster is in actual play.

Omg thanks!!!!! That is Awesome!!! Totally cleared things up! 

I realize I am late to this thread but the main reason CR is not really anyhting more than a rule of thumb is becuase a highly optimized group with a cleric/wizrd/barb/(rogue/bard/fighter/pal/sorc) will be able to easily handle far more combat than a new group with 3 fighters and a druid - so really the answer is while there are rules for cr - its a highly subjective DM call. 

Its on the DM to know the creatures the number of them and the capabilities of the group and to adjust mid combat if needed. 

Quote from SydneyHofferson00762 >>

For  a CR, do you take the level of one character and match the monsters to that level of CR, or do you add up all the PC levels and then match the monsters?

for context, im running a campaign for two lvl5 characters, do i match the monsters to a lvl 10 threat, with a lvl ten monster, or match the CR to lvl ten but using lvl five and below monsters?

Pls give some advice here. I don’t want to kill the characters, but i also don’t want it to be to easy.

Here's a great tool for you, maybe the best: https://koboldplus.club/#/encounter-builder

As for myself, CR is more of a general guide in my games. I find that anything considered as an appropriate CR past level 5 is too easy. So I usually have parties face things 2 CRs higher than the party level (milestone).

For example, a party of 4-5 level 5 characters would face a couple CR7 threats mixed in with the common CR monsters. 

But a level 20 part could face up to CR30 because...well...that's life ;)

In my experience, a monster's cr is about half the equivalent level, so for example, a cr 10 monster is about equal to one level 20 character.

Quote from StereotypicalDungeonMaster >>

In my experience, a monster's cr is about half the equivalent level, so for example, a cr 10 monster is about equal to one level 20 character.

A level 20 character is well over CR 10; effective CR is generally 15-20 depending on build details.

people here tend to have dood answers already but i want to point out action economy, a level 10 player might take down 1 cr 10 creature but 5 cr 2 creatures would be much more likely to kill the player as more turns is very powerful in dnd as well as aoe attacks being more necesary and difficult for martial classes compared to spellcasters.