I do not understanding so much your question (my english is not good)
but, here I come:

For write form, I suggest you to use a template engine

slim is a good one ;)

for get forms values in sinatra, do a post methods and use the params
hash

post '/' do

  puts params['fieldname']

end

The params hash value always contain the request info...

For work with sinatra, ALL you need is in the README

http://www.sinatrarb.com/intro.html


Thread name: "Ruby & Forms" 
Mail number: 1 
Date: Sat, Dec 13, 2014 
In reply to: Bee.Lists 
>
> Hi folks.
> 
> Im trying to get my head around the implementation of Ruby forms.  This is not Rails.  Im messing around in Sinatra and wanted to know how to receive submitted forms.  Ive looked at CGI and the params tag, but Im unclear as to which one should be used.  
> 
> Any urls and insight appreciated as I cant find much out there. Im confused between the two applications and I cant seemto get params to work properly.  
> 
> Cheers