The link @Nicolinux refers to applies to all multi-extrusion printers. If you have a multi-extrusion printer, One At A Time is not supported by Cura.
For a single extrusion printer, you can enable One at a Time printing as follows:
* switch to Custom settings in the sidebar
* in the search bar on top of the Custom settings sidebar, type "print sequence".
* in the Print Sequence setting that is found, change from "All at Once" to "One at a Time"
ahoeben,
THANK YOU, THANK YOU, THANK YOU. Your instructions worked perfectly.
PS: I have a single extrusion printer.
Err, it WAS supported. If I remember rightly when the 1st release of the new software was released, i.e. following 1.5.something, it disappeared but I am sure it was subsequently re-instated. This may all have been before UM produced a dual extruder printer2.0 was released. I have always had a dual extrusion non UM printer and so have always had a custom definition with dual extruders that I can print one at a time with. Now, currently on 2.4, it has been quite a long time since I have printed with multiple models on the bed and it may be that it has disappeared again and I have not spotted it. It was about 10 months ago when I last used it and not sure if I was on 2.4 at that time or an earlier version.
7 hours ago, yellowshark said:Err, it WAS supported. If I remember rightly when the 1st release of the new software was released, i.e. following 1.5.something,
No, not in the new Cura. Cura 15.06 and 15.10 (the infamous "this is the new Cura" releases) as wel as 2.1 did not support multiextrusion at all. Cura 2.3 was the first release to support multi-extrusion, and it disabled One at a Time for multi-extrusion printers.
It is possible (likely?) that 15.04 supported One at at Time for multi-extrusion. It is about just as likely that it could break your printer in doing so.
Fair enough on 15.06 & 15.10, I only used up to 15.04.06. I have just loaded 2.4 and set it up to print one at a time with a multi extruder on a custom printer and everything is fine. The layer no. in the display is 73 and the first 3 copies are "printed" whilst the 4th is part way through. Each model is 20 layers so 80 layers in total. Pic here err no it is not I cannot see anyway of uploading to the gallery/attachments
I think you have specifically turned on the capability to enable the print_sequence setting in your custom printer. You can still do that in Cura 3.1 AFAIK, if you use a custom .def.json printer definition. I would not recommend it for general use, and it is not supported, but it is probably possible.
Anyhow, @KJ1I's question is answered, he is using a single extrusion printer so none of this applies to him.
Edited by ahoeben
Thanks for that @ahoeben. I have not personally created a json file. I just used the Cura menu structure and data items to add my custom printer to Cura. I am not sure how to interpret your first sentence. I just used the special modes/print sequence setting in Cura the same as everyone else has to to print in multi model mode - i.e. to ensure that whichever of the two modes available is set to the one you want.
All I am trying to say is that I have done nothing techie or secret to achieve this. I have just come along as a dumb user and selected the options that Cura provides 
Hi @ahoeben I should add an important point. My memory is vague but when 2 came out a multi extruder printer could not print one at a time; or it may be that no-one could print one at a time. There were a number of people in the same position and there was a dialogue here which included several Ultimaker people. We were told that it would happen in a future release. I like to think that also included dual extruder printers. I cannot be sure without going back and trying to find the thread but all I can say is that I suspect that I would have changed to S3D if that assurance had not been given for dual extruder printers - as I said it is possible that I am wrong. I put this here as I feel a bit uncomfortable having done this by only using Cura 2.4 menus but you have used the phrase "not supported", although I am not sure what that phrase is referring to - presumably not to something that one can do 100% with Cura?
cc @SandervG
The thing is that Cura 2.4 did not, and does not, let you add a custom printer with multiple extruders without doing things with json files. I know this fairly certainly, because I spent quite some time to make this possible in Cura 2.6: https://github.com/Ultimaker/Cura/pull/1521.
So if you are saying you can do one at a time printing with a single extrusion printer in Cura 2.4, I am not contesting that. If you say you have a custom multiextrusion printer in Cura 2.4 that does One at a Time printing, you - or somebody else - created json files for that printer, and specifically enabled the print_sequence setting that is otherwise disabled for all multi-extrusion printers. The code to disable the print_sequence setting for multiextruder printers has been in Cura since november 17th 2016, before Cura 2.4 was released (https://github.com/Ultimaker/Cura/commit/4da9b99ff803e5751af825aefd67cd9346bb93e2)..
I am not sure why you are cc'ing @SanderVG, but Hi Sander!
Hey it says I have to change the value of extruders to be able to change print sequence, how do I do that? Thanks in advance! 
Go to the "Printers" pane of the preferences, select "Machine Settings" and change the number of extruders to 1.
@ahoeben Thanks man! Would never have found it myself
That's why we have the "Ultimaker Community of 3D Printing Experts" ;-)
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Nicolinux 288
It depends on the printer you have set up. Do you use an UM3? If yes, then check this Github issue:
https://github.com/Ultimaker/Cura/issues/3121
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