Hi, 

Yes it is a good idea to mix in compost or aged manure  to your veggie garden patch. Make sure it is aged manure especially if you are getting from a farm, to check that it is ready to apply so it isn't too high in the form of nitrogen where it can burn plants, or still contain possible herbicide residue. 

Adding organic matter helps with beneficial microbes in the soil which can assist with disease issues, and relieve any compaction. It is also a good idea to have you soil tested to see what the pH (acidity level) and nutrient composition is. 

If you have already planted seedlings or seeds, you can wait for them to emerge and top dress or apply a layer in between and around plants. Insects will help incorporate it into the existing soil. If you haven't planted yet, you can add some now. You can gently rake it into the existing soil or spread it over the top and use a garden fork or pitch fork, or even a shovel to 'poke' it in and kind of wiggle the tool back and forth. This will help work it into the soil without damaging the structure by tilling. 

In the future you can apply compost or aged manure in the late fall when you are finished growing things, or over the winter anytime to help replenishing nutrients from the growing season. 

Let us know if you have further questions. 

Emily