Tubes and circles are used a lot in cardmodeling.
There are two different king of joining a tube.
The first is if the designer has "attached" a joining strip at the end of the part like this.


When designing the parts like this we will have a visible edge on the part as I have illustrated on the two pics below.I have put some color to the edge so it's more easy to see.But in this case the part will not be completely round.



So what I do when I'm making a part thet I want the edge to be hidden is to cut of the flip at the end


And I glue it on the inside like this.I have colored the tab so it's more visible.


When the flip is dry I place the part in my hand and with a rounded object I roll up and down the part til it doesnt curve anymore.Thats because the rod I used was a bit big,but I start with a big rod so the paper won't be bent in a way that will show.


Here we can see the maximum curve I got with the end of the knife


So I switch to a smaller tool.Don't be tempted to beleave that you can join the part at this stage.....you can easily bend it instead of curve it nicely.


So when the smaller tool are used we get the proper amount of curve.


And when we glue it together...we get a nice looking tube