Choose your word or phrase or line of poetry or line from a song.
Decide on the approximate height that you want and draw guidance lines on the paper. You will need: one baseline, one for capital letters, one for letters with a tail (g, j, y, p, q), one for the upper limit of lower case letters and perhaps one or two for taller lower case letters, if you don't want these to be the same height as your capital letters (one for t, another line possibly for b, d, f, h, k and l).
Using the lines as a guide, bend your wire to create the letters, holding them up to your guidance lines to check as you go. When you have all of your letters and words, check that they are straight (especially if you have decided to join them together).
If you prefer: Write out your phrase in the size you want it, then bend the letter to follow your written words. When you have all of your letters and words, check that they are straight (especially if you have decided to join them together).
Use the hammer and nails to attach to the wall, placing the nails at appropriate intervals.
The length of wire and time needed will depend on the length of the word(s) or phrase you have chosen. Where you have more than one word, you can decide whether to join the words together (as I did with the homemade wire wall art I made for our anniversary) or whether you want to separate each word (as I did for this Once upon a time example). It is really a matter of personal taste.