Visually hiding a running joint in molding is difficult - so usually it is best to install one continuous strip of molding running the full length of the wall.   

If you are working with existing old molding - and you don't have the dado cutters to match it to make new pieces, then you have to cut the molding and make some type of butt or lap joint. 

The  image above shows the matched cuts on a 5 inch wide gum wood window molding.   This lap can be glued or screwed and then surface imperfections filled.   The resulting joint is invisible once painted.

The image below shows the simple lap cutting jig which allows the 4 inch angled face on the two molding pieces.   To make the cut, hold the molding vertically against the plywood angle guide - starting the feed into the blade from the table saw surface in front of the blade. 

Reverse the face of the molding for the second cut - you will get the result in the top image. 

Wear safety glasses.   Good luck.