I have 2 machines combined into one, I used to do alot of donation afghans on my machine, it the extensions are even a fraction of an inch off they act up. I got a piece of flat straight plywood just a tad bigger than my machine and I mount my machine with that under it. Another tip which maybe hard to understand.
But take a string and anchor it at one end of the machine, run it across the bed of the machine just beside the the needle bar (that little plastic track that holds the needles in where you put the labels on to mark the needles). and achor the string at the opposite end, the string should stay along the bed of the machine, if you notice any spots where the machine is lower than the string, you need to put a shim ( I used some cardboard) until the string is even all the away across.
I know by vision a table top may look straight but the machine finds any flaw.
Also, when using the key plates, don't use anything liquid on them, I use just a bar of soap, to provide enough slide on the plates.
Incase your machine didn't come with it..
The keyplates are normally, 1/2/3/4 sometimes you get the odd .5 plate.
Keyplate 1 is for baby/sport weight yarn (It can be fiddley)
2 is sport and light worsted (I wouldn't go past anything the weight of Bernat Satin.
3 is worsted, all your common yarns, up to something like a Bernat Demin.
4 bulky weight
If you have a ball winder unwind any skeins on your ball winder first before using on the machine, if there is any resistance on the yarn, it shows on the knitting as it tugs a little uneven.
Also row counters are the best ever, esp for sweaters. So if you don't have one, talk to Hilde
If you ever get into it more, and intarsia plate is wonderful for picture knitting.