This is for Mac users who have iPhoto. Sorry, I can't help with any other software or systems, although I'm sure there are various ways to do it on other systems as well. Also, I use the Epson PictureMate Charm, which prints onto 4X6 paper, so most of my tutorial will use that as the example, but you can do this with any printer and on any size of paper.
Select the pics you want to print by clicking on them (command-clicking to select more than one at a time). You will know they are selected when they have yellow borders around them. You can only print one size at a time so if you want your pics all printed at different sizes you will have to print each size separately. I'll start by selecting these 2 pics of my kids playing in the snow:
Then go to the File menu and select "print..."
When you click on print size, you'll get different options depending on the printer and paper size you have selected. The above drop down menu is what I see, but yours may be different. If you want to print any of these size (2X3 is convenient for Project life, for example) just select it here. If you want to print any other size, select Custom:
When you select "custom" the following window opens, where you can set the width and height. I changed mine from 4X6 to 2.5 by 3.5 but it looks like I forgot to get the screen cap of it changed. You'll have to trust me :)
You can see that (2) 2.5 by 3.5 pics fit nicely on a 4X6 photo. This seems to be the biggest pair of pics I can get to fit onto one sheet of 4X6 photo paper, as iPhoto only seems to do borderless printing for 4X6 pics (I could be missing something here though, let me know if you can get 2 4X3s on one sheet). When I print 4X3s I have to print them on separate sheets of paper, or use photoshop elements to create a single 4X6 out of the 2 pics, and print that as a borderless 4X6 (which is way too much hassle for me so I just print 'em separately).
Now, just so you know, if you were using 8 1/2 by 11 paper, this is what the what the window would look like (my two 2.5 by 3.5 pics on a 8.5 X 11 sheet):
...but I'm not, so let's go back to the 4X6 paper :)
If you press the "customize" button to the bottom right of the preview, you get this window:
Here, you can change the cropping (a bit or a lot depending on the aspect ratio of the original pic and the size you are printing it to) and you can also zoom in if you like. Select the picture you want to adjust (it will turn blue) and click the hand icon to drag it around to change the cropping...
Or adjust the slider to zoom the photo in:
By the way, if you had selected more than 2 pics, or more than fits on one sheet, this what you'd see, and you can still adjust the pics on other sheets by clicking it in the top preview area (in the below example I've added a third photo to show what that would look like)
Whenever you're happy with your pics, you can press the print button at the bottom and go through your regular print menu as usual (be sure to print on fine quality to get the best quality prints). By the way in the above window you can also double click the photo at any time to go into the "edit" window where you can make more elaborate changes to the exposure, apply filters, and do more detailed cropping etc. It's the same edit window you would have accessed from your iphoto library at any time. I find this helpful for when I realise the pics I've chosen don't have the same exposure and look too different to scrap together on a layout. Once you're done in the "edit" window be sure to click the "Print menu" button at the top to bring you back to this screen. All the print settings you already set will still be here waiting for you!
I hope this helps. I tried to be as detailed as possible but let me know if anything isn't clear, and I'll try to clarify it. As I said before I'm not into making technical tutorials, so I appreciate any feedback for improving this.