For mounting the batons, invest/borrow a laser spirit level or a long spirit level.
Mount the baton to the wall in a couple of 50-60 cm lengths. Work out the height you want the worktop to be.
Take a tape measure to your current desk if thats about the right height for your chair/working, measure from a fixed point where you know your gonna be consistent, top edge of skirting running measure up a wall for instance, rather than carpet in the middle where you might be at a funky angle.
Note down the height. Deduct the thickness of the worktop, thats the height you want the TOP of your batons.
(Assuming batons are 50mm x 50mm, you want the screws to mount them half way up, so deduct another 25mm. This is your 'drilling holes' height, but will be irrelevant for this method)
Measure up the same edge you originally measure and mark it at the TOP of baton height you calculated earlier (bit of masking tape and a pencil works well if you dont want to mark the walls).
Laser spirit levels are cheap(ish) and great for this next step, mount on a tripod and adjust so that the horizontal laser line runs across the mark you made on the wall. Wander around and mark this line in a number of places (just go around and roughly every fist width put a dot with a pencil.)
Assuming you are mounting into a U shape (3 supporting walls) - Cut your baton into 2 lengths that do the edge of the desk (dont do full length, plan to not come all the way forward, leave 10-15cm, and cut off at 45 degrees to make it look less blocky, avoids smashing your knee into it!). If only supporting on 2 edges (1 long and 1 short) then you only need 1 edge piece.
Cut the pieces to support the long edge, roughly 50-60cm long is fine (makes it easier to handle). Offer up/do the maths and if one needs to be longer/shorter so be it. Wont matter if the worktop is not supported all the way around, so long as its mostly supported.
Plan to leave a gap dead center (assuming thats where the screen is going to go) as it makes it easier to drop the cables down as tight to the wall as you can.
Number your pieces, 1-6 (or whatever), from left to right... So 1 is an end, 2-5 are long edge and 6 is an end. Draw an arrow to show which way up they go.
Get your drill out and (with a wood drill bit thats the correct size for the screw to just about fit through) drill 2-3 holes in each piece, roughly middle and then halfway between the middle and the ends. (I overengineer everything, and have stood on my desk a couple of times...).
Put a masonry drill bit in thats the same size as the wood drill bit you just used.
You may want a friend for this next bit.
Offer the first piece up, paying attention to the dots you made on the wall all being just visible and a consistent distance from the piece. Hold it there and DONT MOVE IT.
Drill with the masonry drill through the holes. Doesnt need to be deep, this is just to mark the wall to show where to actually drill.
Rinse and repeat all the way around with the different pieces, keeping the numbering/order and way up correct.
Swap to the correct size masonry drill for your rawl plugs/plaster plugs, drill the holes and put the plugs in (rawl plugs want to be a bit tight, plaster plugs follow the instructions).
Electric screwdriver time (or tired arm/wrist time) Screws for the plugs, as above I usually overengineer to these tend to be good size and long, 80mm or so (as going though 50mm of baton). Put in, do up loose... Repeat all the way around, keeping it loose.
Spirit level on and check as you tighten, usually a bit of wiggle room!
Worktop will sit on easily if being fitted against 2 walls... if going against 3 walls it can be a bugger and will probably need fettling to fit. Electric plane and a strong friend will be required, as you will be taking it down stairs and outside to plane it... Makes a HUGE amount of dust/mess.
Once its fitting nicely (you want it to drop into place, NOT have to hammer it home... Trust me, been there... Had to cut it to remove it!) drill some holes up through the batons, just long enough to go through but not into the worktop, put some screws in that are longer than the baton is thick and do them up to secure it. Its not going anywhere, but this is just for extra safety!
If mounting on 2 walls and with 1 leg then make sure leg is adjustable and can do the length you need. Follow instructions and mount it. Securing is MUCH more important when its only supported on 2 walls by batons.
Sorry for the essay, but a bit of a step by step seemed wise as have done it 3 times!!
If in doubt, stop and ask for guidance/help.. No point messing it up!
All prices are approx/from memory.
