On the first step, the carbonyl oxygen gets protonated and acquires positive charge.

On the second step, the nucleophilic oxygen of an alcohol donates electrons to the carbon of the protonated carbonyl, bonds with it, and pushes an electron pair of the C=O bond to oxygen, converting it to a C-O bond.

On the third step, a OH-group is protonated by a protonated alkoxy-group.

On the fourth step, the protonated oxygen pulls an electron pair from its bond with carbon, breaks this bond, releases a molecule of water, and generates a carbocation.

On the fith step, the carbocation pulls an electron pair from the adjacent OH-bond, breaks it, releases a proton, and reinstates the carbonyl group.