Right, first off you need to download CDIrip and Toast titanium. Both can be easily found through simple google search. And as far as ISO's go, the best site I have found is http://dcisozone.com/dreamcast-isos.html
Haven't had an issue with a single game i have downloaded.
Right then I will do this step by step.
1) after downloading the iso, unzip it etc
2) open up CDIrip and click on the rip button
3) select the .cdi file that came from the zip.
4)after the rip has completed you should have 1 data file and 1 audio file.
(if you have more than 1 audio file this method won't work)
5) open up toast titanium
6) click on the audio tab (2nd tab on the left of the program)
7) drag the audio file into toast titanium
8) click the big red button and when the options come up click on advanced
9) there should be 2 options write disk or write session
10) make sure WRITE SESSION is selected
11) continue and wait for the burn to complete
12) when it is complete an option will come up to eject or mount. Click mount
13) itunes will then pop up but just cancel anything that appears on screen and minimize itunes
14) click on the tab to the right of the audio tab. (the one with two discs as the image)
15) then in the tabs at the top of the screen go into utilities and select mount disk
16) then select the data file that was produced
17) after this there will be a drop down tab in the middle of toast. click this then the second option should be the data file mounted
18) then click the big red button and this time select WRITE DISC in the advanced settings.
19) let the burning complete then the disc should be good to go!
if you have any problems with this let me know if i can help. also let me know how your attempts go using this method. thanks