Set specific goals. Think what you want to use the language for and decide from there. A personal example:
I'm going back to Japan soon. My specific goals are:
1. Be able to ask room/restaurant related questions in hotels, questions at the car rental office.
2. Vocabulary related to: car: 'unleaded petrol', 'can you fill up the car please?' 'Can you clean the windscreen please?' 'Is it possible to check the tyres please?'
3. Fish, meats - to avoid these foods
4. Train/tube/plane questions involving time, numbers, destinations.
5. Health words and phrases:
I'm hurting...I need...Do you have...?
Some medicine/bodypart words
6. Camera/phone words such as: battery, charger. Whatever you might need or what might crash/break.
7. Post office questions: 'do you have stamps for ....?' 'How much does it cost to send this to...?'
Ideally, ask a friend who speaks the language to write these down for you in our alphabet if it's an Asian language, Arabic or Greek, if you don't know the alphabet.
How will I proceed?
Set a one film a week in the target language as a goal. Or one episode of a series. 'No second chance', an excellent French series adapted from an Harlan Cohen novel, has just arrived on the UK Netflix if you're learning French.
As the trip gets nearer, I will watch more and more Youtube videos in Japanese related to the above, for example, I will type 'bodyparts in Japanese' or 'GPS in Japanese' and will watch them a few times and then onto the next video. This is very important especially the last 2 weeks before going to the country.
I will make lists of words and phrases and will highlight the ones I think I will need the most often, based on my travelling experience.
I will compare my Japanese phrase books and take the best one for my needs.
About one week before leaving, I will write the list of essentials on one piece of paper, which will be in my pocket at all times. It has proven very efficient in the past. Remember that people rarely speak English in Japan, and if you plan to travel outside of Paris in France, most people's English isn't great.
I do this for all my almost all my foreign holidays. When a situation happens, we have to be quick. By the time you have your phrasebook out and find the phrase or word you need, your interlocutor might be gone or...already fed up with waiting.