3 things you will need to create:
1. New gmail
2. Alternate email, I use protonmail.com
3. Textnow (App(Android) or web browser(Any device), I am using web browser on Windows)
I do this in private mode (Chrome) so as to not interfere with any other accounts during creation. Once you have signed in to your new gmail then sign into textnow.com/login using "Log in with Google"
Choose your account on the popup (you might need popup blockers disabled to allow sign in). The next few steps you should be able to figure out by yourself. Once you get to textnow.com/messaging then select "settings" from the menu on left. Change the gmail to the Alternate email, then check that email (it may be in spam) and click email link to confirm. Now on textnow.com/messaging click on Password. Enter anything in the "Old password", then enter your new password in the sections provided, click save. Now this account can be signed in using the Alternate email and password. If you want another account just signout of gmail/textnow and repeat until you have enough textnow accounts.