Buy or download the English version of the vocal synthesizer of your choice. Most -- including Vocaloid and UTAU -- are made in Japan and have incomplete translations, but Cantor is made in Germany and has been fully translated into English.
Install and open the vocal synthesizer. The main interface will take the form of a "piano roll" -- a grid onto which notes can be written. The grid's horizontal axis represents time and its vertical axis represents pitch. In most cases there will be a graphical representation of a piano keyboard with pitch labels along the left-hand side of the interface.
Enter notes on the piano roll grid. This is usually done with the mouse by first selecting a "pencil tool" -- a cursor that looks like a pencil -- and clicking on the grid at the point where the pitch and the point in time that you want the pitch to play meet.
Enter lyrics for your notes. Every vocal synthesizer facilitates the entry of lyrics that it will sing when the "Play" button is clicked. In most cases, there are "English-speaking" voices available for the different vocal synthesizers. Examples for Vocaloid include Prima, Sweet Ann and Big Al. These must be purchased separately. Cantor comes with several English voices built-in. In every case it is possible to enter lyrics by double-clicking on the note to which you want to add lyrics. A text-entry box will appear. Enter the word or syllable that corresponds with the note there.
Adjust the modulation and pitch variance for the notes you have entered. Every vocal synthesizer includes a section below the piano roll grid where many different aspects of the sound can be manipulated. Modulation usually controls volume and/or legato (smoothness and connection of sounds). By creating subtle pitch variances and legato connections between notes, the synthesized voice will sound more natural.
Experiment. Vocal synthesizers are difficult to master and it will take practice to produce natural-sounding vocal lines with them. Read the documentation that came with your vocal synthesizer, if available in English. In the case of UTAU, which has no standard English translation, there are several large online communities in English where advice and tips on technique can be sought. A well-established example is the UTAforum (utaforum.co.cc).