First get rid of the idea that performance appraisals will be objective. They never are no matter how you try becasue objective criteria are generally the worst criteria for job performance because they are rating the things that can be easily measured not the important things. Te single worst evalation systems I ever saw tried to be objective. It failed miserably. It rewarded the mediocre and punished teh best people we had. And it was the easiest to manipulate.
Let's take an example: an objective criteria might be number of lines of code written
Unfortunately, your best guy writes few lines of code becasue he is well able to abstract and write cleaner code. This might mean he is better not worse than the guy who writes lots of code. Further, the really important stuff concerns how difficult the problem was to solve and how well he solved it, neither of which is easily made into an objective criteria.
Now as manager what you should do is keep a file on every subordinate. It should contain any notes about both good things they did and bad things that happened. You can reference this file when doing evals and use your judgement (you are being paid as a manager to use judgement) to determine the relative importance of each. You should know at a minumum what parts of the application they worked on, how well their code worked from a QA standpoint, if they were responsible for any serious production failures, how well they met deadlines, whether they put out extra effort, how well they work with other people, etc.
Anything bad in the file should have been discussed with the employee at the time it happened. In that case look for improements after the problem and look for how they handled fixing the problem. Making the same mistakes repeatedly is a bd thing, learning from mistakes is not. Fixing what you are told to fix is a neutral thing, identifying and proosoing a solution to a problem before someone else found it is a good thing even when the problem is bad. Doing something that brings production down or that the client becomes aware of and escaltes is generally a bad thing.
Now you are almost certainly going to have to rank people when determining pay raises or final eval numbers (much as we would love to be able to do it, not everyone can get the same amount or be rated outstanding), so start out by ranking the employees in your mind after you have read through your notes to refresh your mind.
Now sit dow and write out the most problematic ones first, that is the bad performers and the outstanding ones. These will take the most time and effort. Then write the middle ones. After you have gone through each one, then look at them as a group and make adjustments to make sure that you ahve them ranked correctly. You may need to either change your mental rankings (hey I realized this about that person while I was writing it up) or find a way to differentiate between two people who came out the same but in reality are not the same. This is the time to look for unfair biases in the ranking. Is John really better than Mary or do you just prefer male developers over female ones, might be a question you ask yourself. really consider actual accomplishments iagainst your mental rankings. If they do a self appraisal, it might now be time to look at those and see if how you perceive the person matches up to what they think about themself. If they are far apart be aware that you are going to have a difficult conversation or you might need to adjust your thinking (really if I think you are my best and you think you are only marginally sat, isn't that going to adjust how I think of you?). Be aware that most people rate themselves higher than their actual performance but some less confident ones may go the opposite direction.
Another thing you need to do as a manager is to have the courage to reward good performance and identify bad performance. The worst managers rate everyone close to the same because it is easier to say everyone is highly sat than write up a performance imporevment plan or give one person a bigger raise or bonus. If you are afraid to give someone an honest appraisal because you will then have to do extra work or have an uncomforatble conversation, then you don't belong in management. It is hard to tell someone that their perforncae is unsatisfactory, it is hard to ehar that. But it is far worse to let that be a cancer in your organization becasue you didn;t confront it.