At which point do blocks get validated in the mining process? I'm trying to understand how mining works and I'm stuck on the part about validating blocks and adding them to the blockchain. 

When a miner finds a solution to a block, he shares it with the network, where other miners validate his new block by checking if the hash of the block (using the found nonce) is smaller than the target, right?

At which point does the validation of the block happen and how many users do have to validate the block so it gets appended to the blockchain?

Does this happen before a miner creates a new block? So basically he'd take a new block in the network, validate it, if it's okay, he takes that one as his "previous block" reference?
A block is checked in two places: First, it's checked before a miner starts working on it, and second, it's checked by every other node before accepting the block as valid.