pencil, paper + brain + google?
give us some idea of dimensions, style etc...

I'm a tad confused, I'd think a porch would have a door, a garden fence has a gate...maybe there's a language barrier here?

Reminds me of the old joke about the guy who come home to find his sports car has been painted with white gloss paint.
The handy man says...
'Ive done that painting job you wanted, and by the way, it's not a porch, it's a Ferrari'
Del

Please add the following items to poster #1's suggested list:

A hand (to hold the pencil)

A board to hold the paper

A tape to measure at the site

A scale to measure with on the paper

Thank you,

ps You may also use electronic drafting program in place of the pencil and paper. Sometimes the back of a napkin is substituted for the paper (typically at the pub).

need small gate on porch to prevent baby and dog running off.

I just googled 'garden gates', loads of sites came up, if you look at the pictures, you'll get some ideas.
If you have good woodworking skills you can mortice and tenon the main frame together and then nail on the boards, alternatively you can build the whole thing just nailed together.
Basically they are nearly all a rectangular frame with a diagonal brace and the boards are nailed to that.
Any hardware store will have suitable hinges.
If you build a quick one out of scrap 2x1 you'll get the idea, if it's ok, then fine, or if you want to make a really nice one, you will have the experience to get it right next time.
This is part of the KrisDel^TM WAQAP quality management technique.
(Wrong As Quick As Possible)
Del

How high can the dog jump with the baby on its back?

Do you leave the baby and dog alone on the porch for long periods of time?

Possibly you have some lifestyle issues that you need to change, more than you need to build a gate?

You could get a section of wooden fence, some hinges and a latch. Cut the fence section to fit, hang it on hinges and install the latch. A caster on the open end of the gate will reduce sag. A wire diagonally across the gate would also reduce sag. I did this and it was very easy.

I got a gate from Microsoft, it takes 5 minutes to open.
Del