Ok so there are several ways how to prove your work experience. The ideal would be a signed letter with all needed information directly from your company. However not all are willing to give it away, so in that case you can use different evidence.
You need to prove following:
1. Length of employment (years, months and days).
2. That the employment was paid (and not just formally paid with a very low sum but with some kind of reasonable salary for that position in that country).
3. That you were doing the job tasks according to NOC you are claiming.
So for the point 2 you can use payslips and for point 1 you can use your appointment letter and if ever you got some leaving documents proving when you left your company.
Last point is the most tricky. Some companies might have the work position nicely defined (if they are following some ISO standards for example). However if they have not you can get such letter either from your HR, from your Former manager or from the former colleagues. In this case they have to identify themselves properly and the letter has to be officially singed.
Sometimes companies just do not want to confirm combination of all 3 info together (for example they do not like to reveal your salary). In that case get a partial letter (confirming at least point 3) and then use additional documents to prove the rest.
And if you decide to remove that work experience after ITA and that will change your points, your application might get rejected because you have to have ITA points at the date of ITA and at the date of your AOR (and of course you have to prove it). And in case of your score to be at the cutoff level you need to meet the tie brake criteria. Also be aware that some work experience might not add your CRS points but still does add your FSW points (and this will be checked as well).
