create.your.own.renderer.rb

class RawRenderer < WillPaginate::LinkRenderer
  
  def prepare col, opt, template
    @collection = col
    @options = opt
    @options[:container] = nil
    @template = template
    @total_pages = @collection.total_pages
  end

  def page_link(page, text, attr = {})
    "<a href='/#{root_page}?page=#{page}'>#{text}</a>"
  end

  def root_page
    "items"
  end

  def page_span(page, text, attr = {})
    "<span>#{text}</span>"
  end
end

WillPaginate::ViewHelpers.pagination_options[:renderer] = RawRenderer

inside.sinatra.app.rb

# Use will_paginate, Record is a MongoMapper document class.  This is in your sinatra app:

  require 'will_paginate'
  require 'will_paginate/collection'
  require 'will_paginate/view_helpers'

  helpers do
   include WillPaginate::ViewHelpers
  end

  # note /items is same as #root_page in the renderer
  get '/items' do
    limit = 20
    page = params[:page] || 1
    page = page.to_i
    page -= 1
    offset = limit * page
    records = Records.all :limit => limit, :offset => offset 

    @records = WillPaginate::Collection.create(page + 1, limit, Record.count) {|p| p.replace records }

    erb :your_view
  end

your_view.erb

# just use will_paginate

<%= will_paginate @items %>