What is the easiest program to portablize?

If you have everything needed to get started (described here: https://portableapps.com/development), than I would suggest choosing a small app if you an amateur. I tried to make one once in PA format but I gave up after a long month's work.

Good Luck!

Pick a free open source app you really like so you will have the motivation to stick with it even when you hit the wall during the really frustrating times. For an app check the request apps section. That way users will respond and support your development as they will want to use the app.

....makes programs that have portable versions. All you need to do is download and install the portable version, then create a launcher for it. This is how I got started with NSIS. After that, just pick a cool open source program, look at the source, change it up a bit, and make a launcher for it just like you did with the Piriform ones. If it works, great. If it doesn't, post on here and we'll look at it together, and hopefully get a stable release. To get started, download your programs, read all the forum stickies about development, and take a look at X:\PortableApps\SumatraPDFPortable\Other\Source\SumatraPDFPortable.nsi (where is your drive's letter). This is one of the simplest apps in terms of code, and will get you used to looking at source.