Your costumer wants you to continue a 45 angle that for any reason wasn't finished.   First of all you have to mark a line where the cut has to be done,

after, put a straight piece of wood on the edge of the line and pin it on the wall, so in this way your concrete cutter is going to have a straight line

then start rubbing the material on the sides of the corner with circules and pushing from the corner to the inside the wall

and mark with your pencil a line where the edge of the border guide will be.

for better performance you can add a little bit of grease to your 2x4

Extending an existing 45 angle

Fixing a corner in a tie place

Be sure that your concrete cutter has the same angle of your desired channel, it can be taken from the plastic form or even from another existing channel.

as well, the sides of the corner have to be completely flat, it is always better fill than deal later on with a bump

be sure your level is right

Venetian Plaster in Victoria

Clean all the debris with a broom or preferably with a vacuum, apply an adhesive in the channel

and sharp the corner with a piece of styrofoam

repeat the same on the above line and after cutting the channel, 

Fixing an unpleasant existing 45

apply your concrete material

Fill the whole area with your new concrete and finish it according to your desire, (broom, expose, stamp, etc, finish) remember to keep a good design, if is necessary make another cuts and do the same finishing, in this way is going to looks like it was part of the original design 

cover the whole channel with the adhesive but don't let it get dry, otherwise instead help you to bond the new concrete is going to reject it. Just wait until is sticky and put the new concrete.

this is achieved with a good grinding job, and for example grinding a `45`angle it is necessary  to keep the center of the diamond or cup stone totally center with the `45` angle,

we can make a sample of Venetian Plaster in your project in Victoria and north Vancouver Island

and then you just have to start cutting very slowly and follow the piece of wood. 

Creating a channel

Measure the distance from your "blade to the edge" of the cutter guide in your concrete cutter, pin a 2x4 along the wall measuring from the below line on the wall and the same distance that you took from your "blade to the edge" for example 4 inches from the line below and that is the distance your 2x4 is going to be

in both sides of the corner

It is very, very important, when you want an excellent finishing, specially working in a corner or a "45" angle, that all the bumps have to disappear

one simple technique is to put a layer of material in your styrofoam

and patch it flat

When you face a corner where the space is so tie, you can grind it with a flat disc

chipp any remaining concrete inside and prep your trowel at the same size of your inside channel   

First of all, mark the place where the new cut will be.

Just cut few inches wide and 3/4 deep channel 

just be sure that your disc is a masonry disc and that you put the handle on the grinder

after get rid of all bumps and clean the corner add some water with a water brush

and going up and down with  linear movements until you get a straight line

and start applying the material along the corner in both sides.

then set your concrete cutter in a 45 angle and put it against the wall where the actual 45 is

Fixing a crack on side walk