1> For this we should first confirm the ruby version we are currently on

   rvm current
   (ruby-2.2.3)

2> To view the list of available gemsets before selecting a particular gemset, we can type

 rvm gemset list

 gemsets for ruby-2.2.3 (found in /home/taran/.rvm/gems/ruby-2.2.3)
 => (default)
     global

3> then we can select a particular gemset where ruby is to be installed by using

rvm use 2.2.3@global
(i.e rvm use ruby_version@gemset_name)

(In this case i am installing Rails in the global gemset for ruby-2.2.3)

4> To install specific version of rails in this gemset, type:

gem install rails -v 4.2.0 --no-ri --no-rdoc

('--no-ri --no-rdoc' at the end of above command is optional. This basically prevents the help docs from downloading alongside the rails download. I have done this because its heavy and thus slows down our download process. It's not mandatory, and its completely up to you whether to download or not)