Make your own felt applicators from 5mm sheet. Apply sparingly and carefully so that dye doesnt bleed into the face of leather you may be using one of a less than perfect match to the pre-dyed finish.
Now, here is a bit of advice learnt the hard way: When gluing two pieces of leather prior to stitching; this applies particularly to that with elastic grab properties, avoid at all cost applying so much that when you compress the two faces the gunk oozes from the sandwich. It is equally important to ensure that such does not move within 1mm (ish) of the edge you are now going to sand and burnish smooth etc because you will NEVER conceal such a glue line no mater how hard you sand or have tried to conceal with the dye as at it simply rejects all such efforts. This is why so many apply multiple layers of CONSEALING Edgecoat as it does indeed mask the failure to heed the above or indeed tackle the job as nicely described by Longstrider and would be performed by any old-school leather craftsmen. Just ask Peter Wall, former Master Leather Craftsperson at Cordwainers Colleague, London
Cheers