'Do my bed' is wrong. As a learner of English, you would definitely get 'do my bed' marked incorrect. It would be seen as an error, and you would lose points in a test for writing or saying 'I do my bed' instead of 'I make my bed'. If you were to say this, people would correct you. That said, there might be the odd occasion when you might hear this, especially if making beds is one of a regular set of tasks which someone does as part of their job. For example, if two members of a hotel's staff have to make the bed, sweep the floor, clean the bathroom and refill the tea tray, one might say to the other, "You do the bed and the floor, and I'll do the bathroom and the tea tray". In this situation 'do' stands in for all the other more specific verbs, and it reflects the idea of 'do + a particular task'. [In other contexts, 'do my bed' can also have other meanings. For example, if you've decided to paint all your bedroom furniture pink, you might say 'I've painted my wardrobe and bedside table pink, and tomorrow I'm going to do my bed" (meaning 'paint it'). Context is everything] Those are rare exceptions, however. PS As I was writing this answer, I had the words of another italki member ringing in my ears. A teacher of English from your country recently said to me "Stop bloody confusing people!" She gets angry when native speakers of English tell learners about exceptions, unusual uses of language or (worse still) regional variations. In her view, learners of English should be told only the 'correct' language which they need to pass exams. Maybe she's right. OK, here it is: The basic answer to your question is "It's a big mistake". Don't say it.

It's a mistake, but not a very bad one: a native speaker would understand what you meant, so you would be communicating effectively - but not correctly. The correct expression is, of course, make my bed.

People would only understand it coming from a weak English speaker. If I heard Su.Ki. use it, there’d be uncertainty as to its meaning and I’d wonder if this is another Britishism I didn’t know about. If my American English speaking wife used it, I wouldn’t know what she meant and would probably start worrying about her health.

If a native speaker said "do" he would be saying "I do something to my bed other than making it (because if he were talking about making it he would use the word "make"). In that case, a listener wouldn't know what was meant. On the other had, if someone speaking with a bad accent said "do", we would just assume it was a mistake and interpret it as "make".

In response to Yan's comment: "It's not a big one at all" True. " 'Do' is totally understood by everyone" True. People would know what you meant. "...and often used too" Not true. It isn't used. It's a very common mistake among non-natives, but a native speaker would not replace 'make' with 'do'.