Place the following code in your .vimrc before plug#begin() call

let data_dir = has('nvim') ? stdpath('data') . '/site' : '~/.vim'
if empty(glob(data_dir . '/autoload/plug.vim'))
  silent execute '!curl -fLo '.data_dir.'/autoload/plug.vim --create-dirs  https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
  autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif

Note that --sync flag is used to block the execution until the installer finishes.

If you're behind an HTTP proxy and your proxy does not have TLS/SSL certificates required for Github or if Github has expired TLS/SSL certificates, you may need to add --insecure option to the curl command. In that case, you also need to set $GIT_SSL_NO_VERIFY to true.

Resort to this only if git cannot get updated certificates or if Github has TLS/SSL issues.

You can even go a step further and make :PlugInstall run on startup if there are any missing plugins.

Place the following code in your .vimrc before plug#begin() call

" Install vim-plug if not found
if empty(glob('~/.vim/autoload/plug.vim'))
  silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs
    \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
endif

" Run PlugInstall if there are missing plugins
autocmd VimEnter * if len(filter(values(g:plugs), '!isdirectory(v:val.dir)'))
  \| PlugInstall --sync | source $MYVIMRC
\| endif

Note that this may increase the startup time of Vim.

# vim
vim -es -u vimrc -i NONE -c "PlugInstall" -c "qa"

# neovim
nvim -es -u init.vim -i NONE -c "PlugInstall" -c "qa"

-u is used here to force (n)vim to read only the given vimrc. See :h startup. -i is used here to skip viminfo (vim) or shada (nvim). Reduce the runtimepath via --cmd to speed up vim-plug. Add -V to debug vim-plug if there are errors.

Download plug.vim in autoload directory

curl -fLo ~/.vim/autoload/plug.vim --create-dirs \
    https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim

and update your .vimrc as needed.

filetype off
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()
Plugin 'VundleVim/Vundle.vim'
Plugin 'junegunn/seoul256.vim'
Plugin 'junegunn/goyo.vim'
Plugin 'junegunn/limelight.vim'
call vundle#end()
filetype plugin indent on
syntax enable

call plug#begin('~/.vim/plugged')
Plug 'junegunn/seoul256.vim'
Plug 'junegunn/goyo.vim'
Plug 'junegunn/limelight.vim'
call plug#end()

vim-plug does not require any extra statement other than plug#begin() and plug#end(). You can remove filetype off, filetype plugin indent on and syntax on from your .vimrc as they are automatically handled by plug#begin() and plug#end().

Since all the other major plugin managers store plugins in "bundle" directory, you might want to pass it to plug#begin() if you do not wish to reinstall plugins.

" For Mac/Linux users
call plug#begin('~/.vim/bundle')

" For Windows users
call plug#begin('~/vimfiles/bundle')

Unlike Vundle, vim-plug does not implicitly prepend vim-scripts/ to single-segment argument. So Plugin 'taglist.vim' in Vundle should be explicitly written as Plug 'vim-scripts/taglist.vim'. However, note that vim-scripts.org is no longer maintained.

If you need Vim help for vim-plug itself (e.g. :help plug-options), register vim-plug as a plugin.

Plug 'junegunn/vim-plug'

Use plain "if" statement to conditionally activate plugins:

if has('mac')
  Plug 'junegunn/vim-xmark'
endif

The caveat is that when the condition is not met, PlugClean will try to remove the plugin. This can be problematic if you share the same configuration across terminal Vim, GVim, and Neovim.

" When started with plain Vim, the plugin is not registered
" and PlugClean will try to remove it
if has('nvim')
  Plug 'benekastah/neomake'
endif

Alternatively, you can pass an empty on or for option so that the plugin is registered but not loaded by default depending on the condition.

Plug 'benekastah/neomake', has('nvim') ? {} : { 'on': [] }

A helper function can improve the readability.

function! Cond(cond, ...)
  let opts = get(a:000, 0, {})
  return a:cond ? opts : extend(opts, { 'on': [], 'for': [] })
endfunction

" Looks better
Plug 'benekastah/neomake', Cond(has('nvim'))

" With other options
Plug 'benekastah/neomake', Cond(has('nvim'), { 'on': 'Neomake' })

vim-plug does not natively support installing small Vim plugins from Gist. But there is a workaround if you really want it.

Plug 'https://gist.github.com/952560a43601cd9898f1.git',
    \ { 'as': 'xxx', 'do': 'mkdir -p plugin; cp -f *.vim plugin/' }

With on and for options, vim-plug allows you to defer loading of plugins. But if you want a plugin to be loaded on an event that is not supported by vim-plug, you can set on or for option to an empty list, and use plug#load(names...) function later to load the plugin manually. The following example will load ultisnips and YouCompleteMe first time you enter insert mode.

" Load on nothing
Plug 'SirVer/ultisnips', { 'on': [] }
Plug 'Valloric/YouCompleteMe', { 'on': [] }

augroup load_us_ycm
  autocmd!
  autocmd InsertEnter * call plug#load('ultisnips', 'YouCompleteMe')
                     \| autocmd! load_us_ycm
augroup END