If you mean one like this, I use paint.net. To make it HD you have to change the pixel size. To do that, click on the tab image, then resize. Now, change the resampling to nearest neighbour and un-tick the maintain aspect ratio box. Now, change the both the pixel sizes to 1024, this should result in a 64x64 texture pack. But make sure before you do any of this, use mc patcher to patch it other it will go all funny and weird.