It doesn't get easier....
I make all my safety/parking signs like this....
1) Make the artwork 2mm bigger so 225 x 300mm would be 227 x 302mm
2) Print the signs with margin marks (VersaWorks)
3) On a landscape sign trim the left and bottom edge, so the margin marks are trimmed off. Portrait signs I trim the bottom and right because I would then lay the sign down landscape, if that makes sense.
4) Remove the vinyl from backing and cut off 1/3 of the backing paper
5) Using your thumb and forefinger as guides line up the two edges that you've trimmed flush in the corner (The picture above gives you an idea of where I would be using my fingers as edge guides)
6) You can now move the vinyl until the left side is flush with the vertical edge of the substrate and the bottom is also flush horizontally. (use the backing paper as a guide on the bottom)
7) While holding the vinyl in place let the folded over end roll down on the substrate,
then squeeegeee it down....
8) Using a knife trim the edges
Using this method I make hundreds of parking, safety and road signs a week. Normally I would do all the stages in bulk like trimming the vinyl then laying of the vinyl, then trim them all at once.
It seems simple to me, but having new guys start and teaching them always takes a little time so if you need more instruction let me know and I will make a movie or something. But seriously when doing it in bulk you can make 30-40 signs an hour.