What if I don't know what percentage of weight to use ? If you don't have a 1 rep max or personal best recorded then you can use a RPE system, rating of perceived exertion. The scale for this is 1-10. For example if you are using RPE 7 for 12 reps, this means you have roughly 3-4 reps left in the tank after the 12th rep. If you are using RPE 9, this means you are struggling by your last rep and can MAYBE push out 1 -2 more.
What if the machines are taken when I need to use them ? That's okay! If you need a squat rack or smith machine, go up to the person and ask if you can use the equipment after them. If you are working on accessory work, you can skip around to the next exercise and go back when ready.
What if I don't have enough time ? Shorten the sets! Try to focus on your primary compound lifts and then your accessory lifts. Accessory lifts can be shortened from 4 sets to 2 -3 sets or 3 sets to 2 sets.