My avatar is Reaching Out by Edyta Sitar - it's the second appliqué project I ever did. I think it's fear that's holding you back - just go for it!
If you feel you need to practice machine appliqué, then cut out a few hearts and do those. A heart is a good shape to practice on as it has curves, straight bits and inner and outer angles. You can also then decide what finish you like - satin, blanket or zig-zag and if you want your stitches to show or be invisible. I like to use a rayon in a blending colour so the edges have slight definition.
To centre your block for positioning the pieces fold in half and lightly press - you can half again for quarters.
I would also advise assembling your unit on a silicon sheet before fusing onto the fabric - check out some of the YouTube tutorials.
Good Luck!