If the board was set back a bit to take a skim it would have been faily easy to blend it in. Its hard to tell from the pic if thats the case. I suspect the board has been butted straight up against the existing board and there's been no depth for a skim and would have been better flat jointed. Got to say the edges are rough as a badgers arse and there has been no attempt to finish in the edges.
Not bad mate the actual wall is pretty good and with the blending in always use your brush, keep a bit of “scum” on the side of your bucket for filling hollows plenty of pressure behind the trowel over the scar, like Jess says, you’ve made progress (y)
Try angling the trowel next time along the sections where you're feathering in. There is a knack to it but you should be looking at using the top half of the trowel edge making contact with both the plaster and the existing work. Fair play to you indeed sir!
ive never understood why anyone would plaster half a wall.

I do it sometimes when doing remedial damp work. Tank a section of wall and skim the bit that needed fixing. Saves arsing about prepping the whole wall that has been painted in vinyl.
Did one last week where the bit I didn't skim had window reveals and a boiler enclosure, so saved a lot of time. I don't find feathering that hard, just got to keep an eye on it as it drys and sets. Hate to say it but....it's a lot easier now I've got me sponge!
Try and coat it so it's feathered in, makes it easier, instead of a big lip around the edges. Tidier you coat, easier troweling is. Learn to coat neat,helps massively.
Jess he asked thoughts on his most recent work. To which I answered truthfully.

As for tips i'd say try and get out and work with a plasterer as reading tips online isn't very helpful in this game.
Did you just raise your voice to me...?!:eek: Lol, kidding! That's fair enough bud. And more constructive! ;):D
Make sure and pva out far enough where you are patching into, maybe pva nearly a foot around what you are patching it should feather in easier too.
I was always told if you want to judge the skill of a plasterer check their edges and corners. The field is the easy work so master the blending and the rest will come.

Algeeman was right about doing the whole wall in that situation. You would probably get a better finish but sometimes you can't do that so think of ways to reduce the edges where you can. It looks like you could have gone 1' higher and eliminated 5' of blending at the top. Like I said, not always an option but if it is I'd go that route.

That said I like to use a spray bottle to keep the edge wet and feather it out nice and smooth. Brush/sponge does the same thing but less trips to the bucket with a bottle. Just don't go crazy with the water, you're going for wet and trowelable, not creating a waterfall. Water should never be running down the wall.