You'd be far better as a first timer just cutting a few bricks, you won't need to worry about the joint size, just get them fairly even - measure the first at 10mm and refer back as a visual guide for the rest.
I've cut the bricks on one side of my extension - it makes it easier because any difference along the course in joint/brick size can be taken up by slightly different sized cuts, most are between 17-18cm:
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The other wall is exact bricks length - but I'm always a few mm over or under at the end so it's messing about pushing/pulling the last half a dozen to get them looking ok.
For a course of 20 bricks you should really work to a line, it's as simple as this:
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Set the first brick, transfer the level to the last brick on the other end (spirit level on a straight edge, depending on length you'll probably have to do it in two goes), pull a taut line between the two and off you go.
The proper way to do the beds is to draw the point of the trowel down the middle, cut the excess off the side, butter up your ends etc like on YouTube but you're not a bricky so just get it on however you can and work to the line (as close as you can with the top front of the brick without touching it) - push the brick on firmly, if it's low take it off and add some more mortar, if it's high you can try and tap it down a LITTLE bit with a hammer, otherwise take a bit of mortar out. Don't worry about bits with missing mortar, you'll fill those in when you joint up at the end - just focus on getting that course of bricks on, keep working the mortar and mix in small batches as it'll be going off at the speed you'll be working at.