can anyone tell me how to make custom t-shirts,etc. using my home computer?

Answer 1

You can buy t-shirt transfer paper and make iron-on transfers, then iron them onto shirts.

Or, you can print out big letters in interesting fonts and use them as stencils.
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Answer 2

I am not sure what part you are talking about so I will try and answer the one I think... If you have an image you can upload it to Cafepress.com and buy it from them.. OR you can go to a craft store and buy this special paper you can print it out and then iron it on.

If you are asking how to create an image.. I would google clipart or whatever image you are looking for and pick something that is free for use.