Hello KK,
Like Malik said, you FIRST need to build-up the documentary evidence before you resort to such a step.
Based on my experience, I think it would be better to build up the case right thru the activities he is involved in--rather than giving him a letter that he did this or that [or didn't do this or that].
For eg., since he is into Data-entry, the moment he makes a mistake, let his boss send him a e-mail [which is really pretty convenient, unlike in the past where a Memo is very easily noticed to be the ground for action] pointing the mistakes & ask him to correct them. it could be a very innocuous mail just focusing on the issue. Quite a few times the employee wouldn't see the link between such a mail & the Final objective.
Going by what you mentioned, this process shouldn't take a lot of time--at the most a couple of weeks, at worst & 1 week at best.
When the Chargesheet stage comes, all you need to do is to link such e-mail communications with it & go ahead.
Rgds,
TS