app.rb

require 'sinatra'

get '/process' do
  erb :process
end

post '/process' do
  # whatever with params[:name_input_username]
  ...
  ...
  erb :final
end

final.html.erb

<!-- found in views/final.html.erb -->

<h1>Blah blah blah <%= some_var %></h1>

<h2>Blah blah blah</h2>

process.html.erb

<!-- found in views/process.html.erb -->

<form method="post" action="/process">
	Username:<input type="text" id="id_input_username" name="name_input_username" /><br/>
	<input type="submit" name="Submit"/>
</form>