This Powerpoint is great for discussing initial sounds in words. E.g. in 'tree' the first sound is 't'. Please ensure you practise using the pure sound like 't' rather than 'T' or 'tuh'.
Practise making letters by painting them or make a sensory paint bag! They are less messy and can be reused. Get a plastic, sealable sandwich bag and put some paint in (you could even put some glitter/decorations in) and use a cotton bud or your finger to make marks in the paint. See if they can make patterns or letters in it.