Install

Manual installation: as usual, download it, add to path and set vhd files to require vhdl-tools-mode.

(load-path "...")
(require 'vhdl-tools)
(add-hook 'vhdl-mode-hook (lambda()
                           (vhdl-tools-mode t)))

This package is available fom Melpa too, you may install it by calling

M-x package-install RET vhdl-tools

Finally, you may prefer to use the great use-package instead

(use-package vhdl-tools
  :ensure t
  :defer t
  :config
  (setq vhdl-tools-manage-folding t
        ...))

(use-package vhdl
  :defer t
  :hook (vhdl-mode . (lambda ()
                       (vhdl-tools-mode 1))))