Decide the overall tone and approach your manuscript will take. This will require an honest appraisal of how your work speaks to you. Do you write in a certain narrative style? What kinds of rhythmic techniques or themes seem most prevalent? You are looking for common threads that tie your work together, so the reader has a strong feel for your identity.
Once you have gathered a cross-section of your work, read the possible selections aloud. Listen keenly to how the poems sound as a whole--what reads well on paper may not engage the ear, and vice versa. Whenever possible, separate the strongest contenders into distinct categories. This will give your manuscript a cohesion that it would otherwise lack.
Edit yourself unsparingly. Most poetry collections range from 40 to 80 pages; the old rule of "less is more" applies strongly. You want readers anticipating the next book, instead of waiting for the experience to end. For that reason, always put your strongest poems at the beginning and the end of your collection.
Ask some trusted friends and fellow writers to read your manuscript and critique its potential strengths and weaknesses. Obviously, you want the finished product to reflect your own voice, but someone who is not as close to your work can point out flaws that you may otherwise have overlooked.
Settle the final running order of poems for inclusion. Carefully proofread for grammar and spelling mistakes, which is another good reason to have other people read your work. The last thing you want to convey is slapdash and haste, especially if you are submitting to a publisher.
Combine all your poems into one file. Click the "select all" function of the edit menu on your computer. Then use the copy and pasting functions to assemble the relevant poems into one master file. Numerous software programs are available for the actual typing, but Microsoft Word is the most commonly accepted format.
Use your "Poet's Market" to devise a shortlist of publishers to approach. Read and follow each publisher's submission guidelines carefully--never send a full manuscript if only 10 representative poems are desired. If traditional pieces are the specialty, do not submit accounts of hard-drinking street life.
Assemble a hard copy of your file, as well as a version on disk, for sending your final manuscript to a publisher. Never send your original, if you can help it, and enclose a self-addressed stamped envelope to ensure the safe return of your manuscript. Each publisher has its own response timeline, but two to six months is not unusual.
Think carefully before self-publishing your work. Many poets have gone this route--as Walt Whitman did with his epochal collection, "Leaves Of Grass"--which can pay off, if you have connections that can help review or distribute your work. Just do so with your eyes open, since many of the tasks undertaken by traditional publishers fall on your shoulders.