It's just practice, honestly.
Pick a quiet time of day like early Sunday morning, with good light, no rain, no frost or fog etc. Make sure you have enough petrol.
If you can pick a junction to join the motorway which joins as a 4th lane rather than merges with the left hand lane. As others have said, go at the speed you feel comfortable at, but ideally not much less than 60, unntil you feel comfortable. It's only on motorways that I ever need 6th gear, so it's good to practice that.
Don't hog the middle lane for no reason.
Keep a good distance between you and the car in front. Many drivers (mostly men, I've found) drive far too close to the car in front. If someone pulls into the gap in front of you so that they are too close to you, ease off the gas slightly to increase the safe gap again.
Be aware of junctions coming up as some cars will pull across lanes last minute and suddenly swerve to come off. Also there will be people joining the motorway as you pass too so give them plenty of room between you and the vehicle in front of you so that they can match your speed and get in front of you or behind you safely.
You will need to use your rear view mirror regularly to check for what's approaching from in the distance behind you in general. Some cars go ridiculously fast and whizz out of nowhere (drivers of black Audis, I'm looking at you).
Indicate when you change lanes. So many people don't. Before you change lanes make sure it's safe on both your left and right. People shoudln't undertake but they do. Do a quick glance over your shoulder before you move. Don't pull in too close to any cars that you've overtaken.
The most important thing is - do not panic if you come off at the wrong junction. Most people have a satnav these days so trust it to get you right again.
I find the worst part of driving on motorways is not the getting on and off the slipway but navigating complicated multi lane roundabout juctions with more than one motorway going off them. So many timese these days the paint has gone off the road and you don't know which lane you should be in if you need to go right round. And everyone else seems to know which lane they need to be in as they whizz round. So try to practise those sort of junctions more than once so you are familiar with which lanes you need to be in/switch to before joining the slipway.
Just practice.