Reality of khushoo is not to have any intentional and purposeful thoughts of any thing in the heart other than Allah swt while performing a good deed.

Literally it means absolute stillness/calmness. In the parlance of Shariah it means stillness of the body parts, (which is self explanatory) and also stillness and calmness of the heart. That is, to end all other thinking activities (and focus on Allah SWT).

What is the method to achieve this (i.e. end all other thinking activities and focus on Allah SWT)?

It is not possible to stop all other thinking activities at once immediately. The method is to intentionally focus the attention of the heart towards something desirable. Provided that it does not clash with nature of worship being performed. For example, in salah:
– Focus the attention on Allah SWT, if that is difficult then
– Contemplate that one is facing the holy Ka’ba, or
– Concentrate on the words of Quran and other azkar being read or
– Focus on the meaning of Quran and other azkar being read

This is effective because one can not be attentive towards two things at a single moment. By intentionally and purposefully adopting any of these methods other thinking activities can be halted effectively.

(Basair-e-Hakeemul-Ummat ra 306-7)

However, the guidance from the Sheikh is required in selecting the most effective method for oneself. Also, success or failure in accomplishing it has to be reported to him and based on his feedback one has to achieve the true khushoo required