First, using a screwdriver or chisel, remove any crumbling or loose mortar. You don’t have to remove all the mortar – just anything that’s moving. Once this step is complete, purchase mortar mix and follow the mixing instructions on the package. Use the paste and your best and most expensive tool – your index finger – to fill and patch the gaps. Since you’re repairing a foundation, the color of the mortar might not matter, but in a case where you want it to “match” you can simply use latex paint and an artist’s paint brush to get an exact match.