Any rubbish under the wood can cause errors in your cuts.
Make my own tongue and groove siding.
Depending on how the pieces fit together some jointed sidings can be fully attached by blind nailing through a tongue or underlap that will be covered by the course above.
Insert your new board s tongue into your old board s groove and lay the new board s front groove over the other old board s tongue.
Carefully slice the back part of the groove of your replacement board but leave the front part of the groove attached.
A 1x4 tongue groove pattern is going to create a line every 3 inches while a 1x6 creates its line every 5 inches for the same pattern.
Tongue and groove joints can easily be made with a router when using the right bits on a proper router table.
Check to make sure the siding you choose has a successful track record in your region.
Calculate sqft to lft coverage needed vertical siding applications require horizontal nailers blocking or other wood framing members spaced no more than 24 inches on center.
Make sure your routing table is clear of sawdust and other debris.