You need to make sure that your manuscript is ready. Ask someone to read it and give you honest feedback.
Do research on publishers and find the one that is right for you and your project. Don’t limit the research to being online.
Read and understand the submission guidelines for the publisher you have chosen. The guidelines can usually be found on their websites. Guidelines vary for each publisher, so don’t assume that something that worked for one publisher will work for another.
Make sure your submission is in order. They will want a cover letter; this needs to be professional, concise, and free of spelling and grammatical errors, make it clear why they should publish your book and include a 1-2 line pitch. They will also want a synopsis and this needs to include all key plot points. Your sample material needs to be well-drafted and error free. It also needs to be your finest writing.
Be patient. They will normally have a submission time frame on their website and they will have a tonne of other submissions, not just yours.
Don’t be too hurt if you are rejected. Keep submitting your work and think about ways you can improve it and try to gain feedback from publishers on where to improve.