I am afraid both need correction
The correct spelling of 'itenary' is 'itinerary'
Sentence 1 could be better said this way
1) 'I plan my flight itinerary to Malaysia using t either the shortest or the quickest air route'
( Note; shortest need not be the quickest as it would be dependent on connections)
2) 'When you plan your flight itinerary, please allocate adequate time between connecting flights'
Since the intention is to ensure that the connecting flight is not missed, the word 'spare' has been replaced by the word 'adequate'. Now what will be 'adequate' has to be determined by the circumstances