It is very easy - strip the baby down to a nappy and sit with them in between your legs on the floor, give them something to eat/chew/suck on. Then using a paintbrush paint one foot (then put paintbrush and paint out of babies reach) and then place the babies foot directly onto the canvas placed just in front of you. Press down firmyl but try not to move the foot as you press then pull it up quickly and cleanly (like a plaster!) Wipe the child's foot and do again with the other.
The easiest (though less authentic) way to do it is to paint the hand/foot and then carefully press it onto the canvas from bottom to top and repeat as many times as you need. It means you end up with a great print but it is a bit clinical as you are doing it yourself. Once they get a bit older they are a lot less cooperative though, so at 7 months it would be pretty hard to get them to keep their hand in position and not try to "paint" themselves!
More fun but less precise is to get a big canvas from somewhere like The Works and a couple of ice-cream tubs, fill with different colour paints, encourage baby to put hands/feet in paint (only wearing nappy or unloved clothes as it will get everywhere!) then get baby to crawl/walk/fingerpaint. Awesomely messy but so so fun! If you get a nice day you can do it in the garden then rinse off with hose. Looks quite good if you get a few coordinating colours.
D'oh, by in position I meant that unless you are really quick and precise they will move their fingers/toes automatically, whereas whn they are tiny they don't catch on so quickly and you have a bit longer!
I do hand & footprints onto ceramics at my shop and I use a sponge rather than a brush. If you load with too much paint, you miss seeing all the delicate little lines on their feet so a sponge gives better & lighter coverage.
If you're doing handprints on a younger baby you ideally want a helper. Cover the hand with the paint then flex baby's hand. Hold the very tip of the middle finger back with your own finger as this will make baby open the hand up then pop it down on the canvas (which is being held by your glamorous assistant).