In some respects the birth of uber flexible outfitting in kayaks has killed off the need for the many hours spent shaping and glueing foam together to get that perfect fit. However if you are not using a bean bag footrest or want some outfitting that is a little bit more bespoke then you do need to rely upon cutting/shaping and glueing foam.
The problem with the DIY route isn’t the shaping or even the sticking foam together. The difficulty is often trying to keep foam stuck together or to your kayak after weeks, months or even years of use. The answer is double glueing. The double glue technique requires the use of contact adhesive. You need to full fat version with the nasty fumes. My glue of choice is Evostick, the stuff you get in the red tube/tin. The general instructions suggest that you coat the surfaces of the objects that will be in contact with one another with a thin layer of glue. Allow the glue to go tacky and then press the two pieces together. This does work but the bond isn’t always as strong as it needs to be.
To double glue you coat both surfaces with a thin layer of glue and leave it to go completely dry. You then add another layer to both surfaces and allow that to go tacky before pushing both pieces together, hence the term double glueing. This produces a really strong bond that is unlikely to fail.
This technique works equally well if repairing a raft or attaching D rings to a canoe.