I remove dead bodies professionally full-time for mortuaries/funeral homes. The best way to secure arrangements is to select and contact a mortuary service provider yourself and set up a pre-need. As long as the scene of your passing is obviously a suicide (non-suspicious or possibly linked to a crime against you), what will most likely happen is 911 will be contacted, a medical professional will pronounce you legally deceased, and your county's police coroner will clear your death on-site.
If you have a pre-need, the mortuary is contacted by the police and someone like me comes to pick you up and execute your final wishes (like who gets your remains, or where to scatter your remains). If there is no information available on-site for a pre-need, family will be sought and contacted to have them make at-need arrangements. If no next-of-kin can be found, they will take you to the county coroner's office and store you for a certain amount of time, if you're unclaimed they will handle your cremation at no cost.
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You could also state in your letter or will that you would like X amount of funds from X bank account, or "X amount of cash in white envelope next to the bed" to be probated to direct cremation. Both are legally recognized fund allocations. (edit: In the U.S)