Like riding bycicle... One day you will begin to do all good... Meanwhile, there are many tricks altought difficult to explain with my english lol
Easier to success one... Level up the roller to the top, insert the laminate with a piece of paper and while one person tights it from behind, you lower the roll to the bottom and put the paper/vynil. If its not completely flat, try again.
Another trick on little pieces (up to 2-3 meters) is just fixing a strip of the laminate sheet (not in the roll), and just remove the linear/paper from the laminate while going forward, tensioning at the same time... For me is usually the fastest way for things under 137cms and never fails. The strip you fix has to be completely flat, thats the most important (even more important than more or les tension)
You can use a big piece of 1mm forex/pvc and use it always as starting. Novice people in our shop usually like that way until they get more comfortable with the laminators.
If it has heater you can add up to 60C to make it softer (and make blacks look better)... But well... Not all laminates or vinyls works the same way.
Anyway... You have to look to 2 things... Too low tension will make things like that to you in the main roller. Too much tensión will make like stretched marks just in the vynil (ruining it) but the roller while be tight and will look good.