As someone who can both read and play by ear, I'd say this is a pretty good video on the subject of playing by ear. 

I'd only stress a couple of points.

Firstly, both playing by ear and reading (any notational system) are things where you only really get better at them by doing them.  Don't let yourself get discouraged too easily.  It gets easier as you go along, not harder.  Have some patience with yourself, rather than just saying "Oh, I can't do this.."

Secondly.. I don't personally put any huge importance on "without errors", especially on the first day or two of learning a piece.  Errors can happen, and likely will.  Unless you are in a class and are being graded on "first sight" or "first hearing" playing, they aren't anything to worry too much about. There are not many practical playing/performance situations I can think of where the player will be expected to execute a piece perfectly on their very first attempt to play it after their first exposure to it.  For most of us, at least, we do it that way the "pros" do.  By performing pieces only after we have learned and practised them sufficiently.

Children don't learn to walk without stumbling sometimes.  Most people taking a class don't get 100% on every test.  Errors and learning how to correct them are a part of the learning process. 

But what you really want to avoid is letting the errors go uncorrected. When you find a spot that gives you trouble, stop.  Take the section that is giving you trouble and go over it at a much slower tempo.  If you take it slow enough, even a complex cluster of notes is do-able.  Figure out how slow you have to go to do it right.  Then repeat the phrase several times at that slow tempo to reinforce the notes and build a bit of muscle memory.  Then play it several more times, speeding it up a little each time to find out how fast you can do it without messing up.

Then go back to the start of the piece and play it at that tempo or even a little slower.   Hopefully that will fix it, if not, try the process again.  If it is still a problem, then it can be good to talk to someone who you know could play the piece and see if they can figure out if maybe you are actually doing something "wrong".  There could be some trick of bowing or fingering that you are missing.

If all else fails, turn the problem section into a little exercise to incorporate into your practice sessions as a "drill".  Do it over and over for 5 min or so a day and that can work a lot of the kinks out in a week or less.

The big thing about errors is that you do not want to practice the error itself.  Practice is what builds skill and consistency, and we don't want to get more skilled and reliable at messing up.  Take it slower, maybe break it down into smaller pieces, get some advice from a more experienced player, whatever it takes to be able to get it right.