How do I convert from having two lines to one line so all jacks work on the same line from the dmarc box

If you have a cordless phone, plug into active line. Go to Demarc. You will see 4 post,
Or if newer, a little trickier/ If you remove 1 wire and you still have dial tone, good, remove the 1 next to it, still have dial tone good also. Now those 2 wire that you disconnected need to connect to other pair.
Line 1+ Usually is green or white/blue
Line 1- Usually is red or blue/white

Line 2+ Usually is black or white/orange
Line 2- Usually is yellow or orange/white.

Unless the jacks were wired so some of them have the second line as primary you shouldn't have to do anything. If all the jacks are wired red/green to red/green then the same line is primary. However, if some of the jacks are wired black/yellow to red/green, then the secondary line is primary for that jack so just reverse them.

Easy test for ScottGems is a splitter(L1,L2, L1+L2) Plug phone in each, see if you get dial tone, if you do. Rewiring jack will work. If all jacks before had Line 1 Or Line 2, then demarc may be most quick.